SUMMARY
XP-Pen's Pen/Eraser barrel button changes Brush Preset opacity to be the same as the Pen Mode's Brush Preset Opacity when toggled to Eraser Mode, then when toggled back to Pen Mode, it changes the Brush Preset opacity to 0%.

STEPS TO REPRODUCE
1. Create a New File in Krita. Have XP-Pen's stylus default Pen/Eraser button in the default Pen Mode and select a Brush Preset.
2. Press the Pen/Eraser button to toggle to Eraser Mode then select select an eraser Brush Preset.
3. Press the Pen/Eraser button to toggle back to Pen Mode. The Brush Preset would have it's opacity changed to 0%.
4. Restart Krita.
5. Create a New File.
6. Note the selected Brush Preset's opacity. Press the Pen/Eraser barrel button to go into Eraser Mode. Note how your eraser's opacity is now set to whatever opacity your previous brush was.
7. Use the Pen/Eraser barrel button to toggle back to Pen Mode, again, your Brush Preset will have it's opacity set to 0%.
8. These two behaviors only happens once, every time Krita has restarted.

OBSERVED RESULT
These two behaviors only happens once after starting Krita.

EXPECTED RESULT
Normally switching Brush Presets without changing opacity.

SOFTWARE/OS VERSIONS
Windows: Windows 10 Home, 64-bit.

ADDITIONAL INFORMATION
a) Krita 4.2.8-beta1 (git 0e1e206) 64-bit.
b) XP-Pen Artist 22E pro, Driver Version: XP-PenWin(20191122) Nov 22,2019 PM 14:50
c)This bug has happened in all versions of drivers I tried.
d) This Pen/Eraser button toggles to Pen Mode and Eraser mode just fine in Clipstudio Paint and Leonardo Beta.