Created attachment 121580 [details]
Loss of data in animated frames of the transparency mask

SUMMARY
Transparency mask lose content

STEPS TO REPRODUCE
1. Create a transparency mask from a duplicated animated layer, or import animated frames, in order to mask it.
2. The layer keeps its animated frames and masks the layer appropriately, we save the file.
3. When you open the file, the transparency mask will lose its content leaving only empty frames

OBSERVED RESULT
The transparency mask works as expected, reproducing next to the animation, the failure comes when retrieving the information when reopening the file

EXPECTED RESULT
When opening the file you should be able to keep the data of the transparency mask and all the frames.
(in some previous version prior to 4.0 the result could be obtained)

SOFTWARE/OS VERSIONS
Windows: 7
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
KDE Frameworks 5: LGPLv2.1+
Qt Version: 

ADDITIONAL INFORMATION

Files with similar data created in previous versions do not lose the information in layers of transparency as in the last versions, although when retrying recreating in newly downloaded versions, data loss still suffers