SUMMARY
Using the 4.3.0-prealpha appimage (git 05ad360), I had initial difficulties with the Mixer Slider docker plugin. I didn't know that a restart was needed.

STEPS TO REPRODUCE
1. Use the Python Plugin Manager to enable the Mixer Slider docker or the Assign Profile to Image docker (as examples).
2. Follow the instructions for their use as given when clicking/selecting the name of the plugin in the list of plugins.
3. Become confused when they don't work.

OBSERVED RESULT
It seems that you need to restart krita to have them working.

EXPECTED RESULT
A note about this, next to the title 'Python Plugin Manager' would be very helpful and there is room to put one there.

SOFTWARE/OS VERSIONS
Krita

 Version: 4.3.0-prealpha (git 05ad360)
 Languages: en_GB, en, en, en_GB, en
 Hidpi: true

Qt

  Version (compiled): 5.12.5
  Version (loaded): 5.12.5

OS Information

  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 4.19.0-6-amd64
  Pretty Productname: Debian GNU/Linux 10 (buster)
  Product Type: debian
  Product Version: 10

OpenGL Info
 
  Vendor:  "NVIDIA Corporation" 
  Renderer:  "GeForce GTX 750 Ti/PCIe/SSE2" 
  Version:  "4.6.0 NVIDIA 418.74" 
  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
  Undo Enabled: 1
  Undo Stack Limit: 18
  Use OpenGL: 1
  Use OpenGL Texture Buffer: 1
  Use AMD Vectorization Workaround: 0
  Canvas State: OPENGL_SUCCESS
  Autosave Interval: 360
  Use Backup Files: 1
  Number of Backups Kept: 1
  Backup File Suffix: ~
  Backup Location: Same Folder as the File
  Use Win8 Pointer Input: 0
  Use RightMiddleTabletButton Workaround: 0
  Levels of Detail Enabled: 0
  Use Zip64: 0


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: 60