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SUMMARY
On Krita 5.2.2, trying to add keyframes to the Animation Curves is buggy.
This is true on Windows 10 and Windows 11.
I checked an old version (I believe 5.0.2) and the "Add Keyframes" button worked as expected. After updating, it did not.
Most of the time, creating a blank keyframe for a Transform Mask property will work on the first keyframe, but when scrubbing the playhead to another frame and clicking "Add Keyframe", it will not, or may add a keyframe in a different frame altogether, but most of the time will not add any additional keyframes and is buggy.

STEPS TO REPRODUCE
1. Make a drawing on a layer, right-click the layer, Add > Transform Mask. 
2. With the Animation workspace active on frame 0, go to Animation Curves. With the Transform Mask layer active, click the Add Keyframe button (looks like -.- with a +). 
3. Scrub the timeline (orange drag n drop bar) to another frame. Click the "Add Keyframe" button again. No new keyframes will be added. In version 5.0.2, new keyframes could be added freely, no problem, as expected. Now it is very buggy. 

OBSERVED RESULT
Instead of adding keyframes anywhere in the timeline, either no new keyframes will be added, or they may be added randomly to frames near where the playhead is, or they very rarely will be added where expected.

EXPECTED RESULT
That new keyframes will be added for each property precisely where the playhead is at in the timeline, for the selected layer.

SOFTWARE/OS VERSIONS
Windows: 10 (_86x64) and 11 (likely also _86x64)
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
I think it was version 5.0.2 that worked seamlessly and as expected.