SUMMARY
While creating a new Fill layer, if you switch to Multigrid and then quickly switch to another option like Pattern, you'll get a malformed preview showing both types.

STEPS TO REPRODUCE
1. Add Fill layer
2. Switch to Multigrid
3. Quickly after step 2, switch to another option.

OBSERVED RESULT
The rendering preview is malformed. See attachment.

EXPECTED RESULT
The option you switched to in step 3 should be correctly shown.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Debian sid
(available in About System)
KDE Plasma Version: 5.17.5
KDE Frameworks Version: 5.70.0
Qt Version: 5.12.9 (Appimage)

ADDITIONAL INFORMATION
This happens regardless if it's CPU canvas or OpenGL canvas.

This only happens when creating the fill layer, not when editing the properties of an already created fill layer.

I marked this as minor as the final render comes out fine regardless of how the preview looks.

If step 3 is not done immediately after step 2, the preview looks normal.