Bug 452170

SUMMARY
This is the case for all versions and is reported for the Apr 01 5.1.0-prealpha (git a005597297) appimage on Debian 10.

A testing file, 'Manipulate Anim Trans-Filter Masks', is attached for ease and convenience of testing.

This has a static painted rectangle which has a simple animated transparency mask on it.
It also has an animated Hue shift Filter Mask on it with the same painted content.
There is also an animated paint layer that has content similar to the animated masks, for comparison.

Note: Any testing will be subject to this bug:
https://bugs.kde.org/show_bug.cgi?id=452153
so that if you close the file then open it again, there will probably be a crash.

STEPS TO REPRODUCE
1. Open the attached file, enable the Trans-1 mask and observe its contents on the Timeline.
2. Select a frame of Trans-1 and do Copy Keyframes.
3. Select frame-30 (or somewhere) of Trans-1 and try to Paste Keyframes.
4. Select frame-30 of Paint-2 and try to Paste Keyframes.
5. Try the preceding steps but using the Filter-1 mask layer and frames.
6. Try Mirroring and select-dragging frames of the masks.

OBSERVED RESULT
2. It can be copied
3. It can't be pasted, the action is greyed out.
4. It can be pasted onto the paint layer frame.
5. You can't paste onto a filter mask keyframe.
6. That can be done.

EXPECTED RESULT
An animated transparency mask and an animated filter mask should be able to be manipulated on the Timeline in the same way that an animated paint layer can be manipulated.

SOFTWARE/OS VERSIONS
Krita
 Version: 5.1.0-prealpha (git a005597)
 Languages: en_GB, en, en, en_GB, en
 Hidpi: false

Qt
  Version (compiled): 5.12.12
  Version (loaded): 5.12.12

OS Information
  Build ABI: x86_64-little_endian-lp64
  Build CPU: x86_64
  CPU: x86_64
  Kernel Type: linux
  Kernel Version: 4.19.0-20-amd64
  Pretty Productname: Debian GNU/Linux 10 (buster)
  Product Type: debian
  Product Version: 10
  Desktop: MATE