SUMMARY
This problem happens with appimages and does not happen with Windows builds.
It has been observed with a Debian 9 (Stable, fully updated) system using a Wacom Intuos Paint tablet (CTH 490/K).
It's inconvenient but it's not a major problem. It happens on Debian 10 (Testing but under full freeze) in a different way as noted below.
This was tested with the 4.1.7 appimage and the latest (15th May) 4.2 alpha appimage and Windows versions.

When changing from drawing with a pen to drawing with a mouse, the mouse does not draw and the brush profile outline does not track the cursor. The cursor must be moved off the workspace and then back to the canvas, as a 'recovery method' in order to get the brush outline moving. After the cursor moves off the workspace, the brush outline tracks the cursor as it returns to the workspace. A single mouse click is needed (on the workspace border area or the canvas) to get the mouse drawing again. Further, a mouse click before moving off the workspace will result in that extra single mouseclick not being needed after the cursor is brought back onto the canvas.
Other tools such as line tool, colour picker and selection tools show the same mouse action failure and 'recovery method' characteristics.

Note: With the 4.1.7 appimage, the additional mouse click is not required to enable drawing with the mouse after tracking has been recovered in the way described above.

In Debian 10, the brush tracks the mouse after switching but a click on the canvas is needed before it will draw.

In ADDITIONAL INFORMATION, tablet tester outputs for stylus to mouse transition are shown for different krita versions and different operating systems. They are all different. 


STEPS TO REPRODUCE
1. Create a new image and zoom out a little to show a border area (for convenience). Use the pen and Freehand Brush to draw with any brush preset. Remove the pen from the tablet area.
2. Move the mouse and attempt to draw. Note the lack of drawing and lack of brush outline response to the cursor.
3. Move the mouse off the workspace, e.g to an adjacent docker or the toolbar. Note the brush profile outline will be removed from the canvas and will now track the cursor as it returns to the workspace and the mouse will not draw on the first attempt unless it is clicked on the workspace border or canvas area first.
4. Also note that switching to drawing with a pen at any stage does not show any problems but then switching back to the mouse repeats the mouse drawing problem situation.

OBSERVED RESULT
As noted in STEPS TO REPRODUCE above.

EXPECTED RESULT
The mouse should be able to draw on the canvas after use of the pen.

SOFTWARE/OS VERSIONS

Krita

 Version: 4.2.0-alpha (git 7a773b1)
 Languages: en_GB, en
 Hidpi: true

Qt

  Version (compiled): 5.12.2
  Version (loaded): 5.12.2

OS Information

  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 4.9.0-9-amd64
  Pretty Productname: Debian GNU/Linux 9 (stretch)
  Product Type: debian
  Product Version: 9

Hardware Information

  GPU Acceleration: auto
  Memory: 16049 Mb
  Number of Cores: 8
  Swap Location: /tmp


ADDITIONAL INFORMATION

Tablet Tester: 4.2 alpha appimage Debian 9
---------------------------------------------------
Stylus press X=343.46 Y=19.28 B=1 P=4.0%
Stylus move X=344.64 Y=18.31 B=1 P=4.9% (DRAW)
Stylus move X=345.32 Y=17.45 B=1 P=5.0% (DRAW)
Stylus move X=346.16 Y=16.69 B=1 P=5.1% (DRAW)
etc
Stylus move X=347.67 Y=17.45 B=1 P=2.6% (DRAW)
Stylus move X=348.18 Y=17.45 B=1 P=0.0% (DRAW)
Stylus release X=348.18 Y=17.45 B=0 P=0.0%
(switch to mouse)
Mouse press X=305 Y=30 B=1
Mouse move X=305 Y=31 B=1
Mouse move X=302 Y=35 B=1
etc
Mouse move X=301 Y=43 B=1
Mouse move X=301 Y=44 B=1
Mouse release


Tablet Tester: 4.1.7 appimage Debian 9
-----------------------------------------------
Stylus press X=273.00 Y=19.41 B=1 P=16.3%
Mouse press X=272 Y=19 B=1
Stylus move X=272.16 Y=19.62 B=1 P=25.7% (DRAW)
Stylus move X=271.32 Y=19.84 B=1 P=29.3% (DRAW)
Mouse move X=271 Y=19 B=1
Stylus move X=270.64 Y=20.05 B=1 P=30.9% (DRAW)
Mouse move X=270 Y=20 B=1
etc
Stylus move X=265.93 Y=21.13 B=1 P=36.8% (DRAW)
Mouse move X=265 Y=21 B=1
Stylus move X=265.42 Y=21.56 B=1 P=20.3% (DRAW)
Stylus move X=265.42 Y=21.56 B=1 P=0.0% (DRAW)
Stylus release X=265.42 Y=21.56 B=0 P=0.0%
Mouse release
(switch to mouse)
Mouse press X=212 Y=71 B=1
Mouse move X=212 Y=72 B=1
Mouse move X=211 Y=75 B=1
etc
Mouse move X=207 Y=80 B=1
Mouse move X=207 Y=81 B=1
Mouse release

Tablet Tester: 4.2 alpha Windows 10
------------------------------------------------------
Pen tip brought near
Stylus press X=291.29 Y=26.20 B=1 P=15.3%
Mouse press X=291 Y=26 B=1
Stylus move X=290.62 Y=26.41 B=1 P=23.7% (DRAW)
Mouse move X=290 Y=26 B=1
Stylus move X=289.95 Y=26.52 B=1 P=27.1% (DRAW)
Mouse move X=289 Y=26 B=1
Stylus move X=289.44 Y=26.52 B=1 P=32.3% (DRAW)
etc
Stylus move X=271.08 Y=35.36 B=1 P=47.9% (DRAW)
Stylus move X=271.08 Y=34.93 B=1 P=22.6% (DRAW)
Mouse move X=271 Y=34 B=1
Stylus release X=271.08 Y=34.93 B=0 P=0.0%
Mouse release
Pen tip taken away
(switch to mouse)
Mouse press X=219 Y=96 B=1
Mouse move X=218 Y=101 B=1
Mouse move X=218 Y=102 B=1
Mouse move X=218 Y=104 B=1
etc
Mouse move X=214 Y=112 B=1
Mouse move X=214 Y=113 B=1
Mouse move X=213 Y=113 B=1
Mouse release

Tablet Tester: 4.2 alpha appimage Debian 10
--------------------------------------------------

Pen tip brought near
Stylus press X=254.26 Y=28.67 B=1 P=17.1%
Stylus move X=252.75 Y=29.53 B=1 P=19.5% (DRAW)
Stylus move X=251.23 Y=30.29 B=1 P=19.8% (DRAW)
Stylus move X=249.72 Y=31.15 B=1 P=20.5% (DRAW)
Stylus move X=248.71 Y=31.69 B=1 P=24.5% (DRAW)
etc
Stylus move X=242.31 Y=38.80 B=1 P=2.4% (DRAW)
Stylus move X=242.31 Y=38.80 B=1 P=0.0% (DRAW)
Stylus release X=242.31 Y=38.80 B=0 P=0.0%
Pen tip taken away
(switch to mouse)
Mouse press X=271 Y=63 B=1
Mouse move X=271 Y=63 B=1
Mouse move X=271 Y=64 B=1
Mouse move X=269 Y=70 B=1
etc
Mouse move X=260 Y=97 B=1
Mouse move X=259 Y=99 B=1
Mouse move X=257 Y=100 B=1
Mouse release