SUMMARY
In the latest 4.3.1 alpha appimage (git 040ab1a), the watercolour brush presets have their on-canvas outline as a square and many do not paint well.
The same presets, if saved as new presets, work fine in the 4.3.0 appimage.
The 5.0.0 prealpha shows the same behaviour.

I believe that this is not related to:
https://bugs.kde.org/show_bug.cgi?id=423359

Turning layer visibility on/off and Save/re-Open show no change.

Turning canvas graphics acceleration on/off shows no changes (except that the brush outline is purple instead of black with cga off but this seems to be a new thing for all preset outlines).

STEPS TO REPRODUCE
1. Run 4.3.0 and note the appearance and behaviour of the Watercolour brush presets.
   Especially observe j)_WaterC_Basic_Round-Fringe_02 and j)_WaterC_Flat_Big-Grain_Tilt as examples.
2. Repeat Step 1. but running the 4.3.1 alpha appimage.
3. Save new presets for j)_WaterC_Basic_Round-Fringe_02 and j)_WaterC_Flat_Big-Grain_Tilt
4. Run 4.3.0 and check the new saved watercolour brush presets.

OBSERVED RESULT
1. They look good and they work (but some need a stylus rather than a mouse).
2a. Most have a square brush outline but use the same/correct brush tip as in 4.3.0 and they paint in black. not the selected colour.
2b. j)_WaterC_Basic_Round-Fringe_02 and j)_WaterC_Flat_Big-Grain_Tilt do not paint well but have the same/correct brush tip as in 4.3.0
4. The new saved watercolour presets behave well.

EXPECTED RESULT
Those brush presets should work as they do in 4.3.0.

SOFTWARE/OS VERSIONS
Krita

 Version: 4.3.1-alpha (git 040ab1a)
 Languages: en_GB, en, en, en_GB, en
 Hidpi: false

Qt

  Version (compiled): 5.12.9
  Version (loaded): 5.12.9

OS Information

  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 4.19.0-10-amd64
  Pretty Productname: Debian GNU/Linux 10 (buster)
  Product Type: debian
  Product Version: 10
  Desktop: MATE

OpenGL Info
 
  Vendor:  "NVIDIA Corporation" 
  Renderer:  "GeForce GTX 750 Ti/PCIe/SSE2" 
  Version:  "4.6.0 NVIDIA 440.100" 
  Shading language:  "4.60 NVIDIA" 
  Requested format:  QSurfaceFormat(version 3.0, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::DoubleBuffer, swapInterval 0, colorSpace QSurfaceFormat::DefaultColorSpace, profile  QSurfaceFormat::CompatibilityProfile) 
  Current format:    QSurfaceFormat(version 4.6, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 8, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::DoubleBuffer, swapInterval 0, colorSpace QSurfaceFormat::DefaultColorSpace, profile  QSurfaceFormat::CompatibilityProfile) 
     Version: 4.6
     Supports deprecated functions true 
     is OpenGL ES: false 

QPA OpenGL Detection Info 
  supportsDesktopGL: true 
  supportsOpenGLES: true 
  isQtPreferOpenGLES: false 


ADDITIONAL INFORMATION