Created attachment 152503 [details]
This is a demo project that I was able to replicate the bug in. I tried to make it as simple as possible to isolate the problem.

SUMMARY
"Paste Into Active Layer" works as expected on the first frame of an animation, but on any other frame, it will copy the contents of the first frame onto the current frame instead of working only with the contents of the current frame.

I've included a demo file that you can use to replicate the bug.

STEPS TO REPRODUCE
1. Go to frame #19 of the demo file and select the squiggle layer.
2. Use lasso tool to select part of the squiggle.
3. Edit->Copy to copy the selection
4. Edit->Paste into Active Layer to paste the selection.

OBSERVED RESULT
The entire contents of the first frame of the animation are pasted into the active layer.

EXPECTED RESULT
The selected portion of the current frame is pasted into the active layer.

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

ADDITIONAL INFORMATION