Toolbar button image vanishes and doesn't come back

(8.0.23125.12175, 2023-05-05)

I installed a RUI file that works fine in V7.
It has pixel based icons.
Displays ok in V8 for now.

When I play with the light/dark settings, the icon disappears.

Ok, V8 uses SVG, so better don’t try to change the pixel icon for now.
I replaced the destroyed RUI file with a working, unchanged one.
The icon should be back, but it isn’t.

Next try:
I delete the RUI and start V8.
My RUI isn’t there, fine.
Shut down V8.

Installed my working RUI file.
Started V8.
Icon is still not there.
It must be stored somewhere I don’t know.

_Reset in V8.
Started V8, shut down V8.

Installed my RUI file.
Started V8.
Icon is there.

Will there be a cure for handling pixel icons?