If you create a group layer and drag and drop a new layer into it, there is no issue. However the bug arises when you already have layers added to a group layer that is collapsed. 

If I have a layer that I want to add to an existing group then I must expand the group layer in order to drop the new layer into it. This isn't intuitive as the user expects to be able to drop layers into a group layer whether it is expanded or not, and seems to be a bug as the user can drag and drop the initial layer into the group as expected.

STEPS TO REPRODUCE
1. Create a group layer, create a new layer (Layer 1) and drag and drop a layer into the group layer
2. Collapse the group layer
2. Create a new layer (Layer 2)
3. Drag the new layer (Layer 2) onto the collapsed group layer

OBSERVED RESULT
The new layer (Layer 2) is not added to the group layer

EXPECTED RESULT
The new layer (Layer 2) is added to the group layer

Operating System: KDE neon 5.26
KDE Plasma Version: 5.26.0
KDE Frameworks Version: 5.99.0
Qt Version: 5.15.7
Kernel Version: 5.15.0-53-generic (64-bit)
Graphics Platform: X11