SUMMARY
Set any assisstant tool on canvas, assign it a custom color, change opacity from 100% to anything (different from 0, 20, 40, 60, 80, 100 percent), click on hide (eye) or move icon of the assi. tool on cavas, the opacity of this assisstant gets lowered everytime you use either down to multiples of 20% so if you set opacity to 98% and click hide or move on the assistant panel the opacity will soon drop to 80%, if you set it to 30% it will drop to 20% in a few clicks, if oy uset it to 18% it will eventually drop down to 0%.

It works this way only with setting custom color.

STEPS TO REPRODUCE
1. Put any assistant tool on canvas.
2. Assign it a custom color.
3. Change the assistant's opacity to anything that's not an integral multiple of 20% (for example set it to 95%).
4. Hide and unhide (eye icon) the assistant or move it a few times.

OBSERVED RESULT
Opacity of the custom color drops down to the nearest integral multiple of 20% (from 95% few clicks drop it down to 80%).

EXPECTED RESULT
Opacity should stay set to a desired value without any change to it by moving or un/hiding the assisstant tool.

SOFTWARE/OS VERSIONS
Krita

 Version: 4.2.5
 Languages: cs, en_US
 Hidpi: true

Qt

  Version (compiled): 5.12.4
  Version (loaded): 5.12.4

OS Information

  Build ABI: x86_64-little_endian-llp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: winnt
  Kernel Version: 10.0.17134
  Pretty Productname: Windows 10 (10.0)
  Product Type: windows
  Product Version: 10

SIDE NOTE
I also tested this on the latest bug fix nightly build, the same problem occurs.