Created attachment 154242 [details]
When I saved the file, the layers were behaving badly. The file behaves normally when opened.

SUMMARY
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I hit this bug while following the tutorial here: https://www.youtube.com/watch?v=a6Sg25lfoIE. I was not able to reproduce it a second time after restarting Krita. Here is what I remember about the steps I took.

STEPS TO REPRODUCE
1. Start Krita, create a new 2000x2000 document
2. Use Basic-1 brush and set size to somewhere around 800px using Shift+mouse drag. Tap on the canvas to make a circle. Set color to a medium-value desaturated blue and use Fill to fill the circle.
2. Ctrl+Shift+G to create a clipping group. In the new mask layer, use airbrush at 1000px with dark desaturated purple to draw shading around the bottom and right of the circle. Toggle eraser mode and use it lighten a bit of the shadow around the middle of the circle. Change layer opacity to around 75%.
3. Right-click the mask layer and duplicate it. Un-toggle eraser mode. Set color to pale desaturated yellow. In the new mask layer, attempt to draw highlight along top left edge of the circle with the airbrush.

OBSERVED RESULT
Nothing is drawn. Even the mask layer's thumbnail doesn't show any content in the layer. The brush will only draw on top of the crescent shadow from the original mask layer, even though the new mask layer should also inherit alpha from the circle layer at the bottom of the clipping group.

EXPECTED RESULT
Airbrush should be able to draw on top of any part of the circle.

4. After this, I tried some other things and got more weird behavior. Unfortunately I am not sure of my exact repro steps after this point.
4a. I deleted the new mask layer and tried to perform the tutorial steps again. Once again, I duplicated the mask layer in the clipping group, cleared the new layer, and found I could not draw on it.
4b. I duplicated Paint Layer 1 (called Layer 1 in Kai's tutorial) and moved it outside the clipping group, above the background layer. When I drew on it, somehow it was inheriting its *own* alpha--I could only draw inside the circle that was already there.
4c. The original mask layer started inheriting its own alpha as well--when I tried to draw on it with a brush, I could only recolor the crescent shading on that layer.
4d. The duplicated mask layer wouldn't let me draw anything anywhere. Possibly it was also inheriting its own alpha, and it was completely transparent already.

5. Finally, I saved the file, restarted Krita, and re-opened the file. All the layers were now working as expected and I was able to follow the tutorial.

6. I created a new file and performed the same (as close as I could remember) initial steps again, but I couldn't repro the bug a second time.

SOFTWARE/OS VERSIONS
Windows: 22H2 (OS Build 22621.900)
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
The attached file is the file I saved in step 5. Unfortunately it doesn't repro the bug when it's opened.