SUMMARY
This crash was observed with all fresh configs and resources folder.
It does not affect the 4.2.9 appimage.
It affects the 4.3.0 beta-1 and all future appimages including the 26May 5.0.0 prealpha (git 52ec7fd)


STEPS TO REPRODUCE
1. Start krita and create a new document 2048 x 2048
2. Create another new document 2048 x 2048
[Note the vanishment crash]
3. Restart krita and create a new document 1024 x 1024
4. Create a second new document 1024 x 1024
[Note that there is no crash]
5. Create a third new document 2048 x 2048
[Note that there is no crash]
6. Create a fourth new document A4(300ppi)
[Note the vanishment crash]

OBSERVED RESULT
See Steps To Reproduce

Running from a terminal gives the following crash result:
=================================
qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 6180, resource id: 12669652, major code: 40 (TranslateCoords), minor code: 0
ASSERT (krita): "d->canvas == canvas" in file /home/appimage/workspace/Krita_Nightly_Appimage_Build/krita/libs/ui/input/kis_input_manager_p.cpp, line 239
Aborted
================================

The strange thing is that if you first create an A4(300ppi) document, the a 2048 x 2048 document, there is no crash and you can then create another A4(300ppi) document.

EXPECTED RESULT
It shouldn't crash

SOFTWARE
Krita

 Version: 5.0.0-prealpha (git 52ec7fd)
 Languages: en_GB, en, en, en_GB, en
 Hidpi: true

Qt

  Version (compiled): 5.12.8
  Version (loaded): 5.12.8

OS Information

  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 4.19.0-8-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 440.82" 
  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 

Hardware Information

  GPU Acceleration: auto
  Memory: 16039 Mb
  Number of Cores: 8
  Swap Location: /tmp

Current Settings

  Current Swap Location: /tmp
  Current Swap Location writable: true
  Undo Enabled: true
  Undo Stack Limit: 30
  Use OpenGL: true
  Use OpenGL Texture Buffer: true
  Use AMD Vectorization Workaround: false
  Canvas State: OPENGL_SUCCESS
  Autosave Interval: 900
  Use Backup Files: true
  Number of Backups Kept: 1
  Backup File Suffix: ~
  Backup Location: Same Folder as the File
  Backup Location writable: false
  Use Win8 Pointer Input: false
  Use RightMiddleTabletButton Workaround: false
  Levels of Detail Enabled: true
  Use Zip64: false

Display Information
Number of screens: 2
	Screen: 0
		Name: DVI-D-1
		Depth: 24
		Scale: 1
		Resolution in pixels: 1280x1024
		Manufacturer: Dell Inc.
		Model: DELL 1704FPV-
		Refresh Rate: 60
	Screen: 1
		Name: DVI-D-0
		Depth: 24
		Scale: 1
		Resolution in pixels: 1280x1024
		Manufacturer: Dell Inc.
		Model: DELL 1704FPV-
		Refresh Rate: 75