Fedora icon gone from the panel after upgrade to F40

I’ve upgraded a couple of laptops to Fedora 40. On one of them, the icon to the very left in the panel is the stock KDE/Plasma icon (three dots and a greater than sign) rather than the fedora Icon it had in F39, and which is shown on the other. There are certainly differences which I would assume irrelevant for this. The one misbehaving is e.g. normally connected to an external large monitor. But I can’t imagine what I may have done to cause this difference.

Does anyone have a guess what might cause this?

I don’t know if it is related but I’ve found that since KDE moved to Qt6 icon themes must be “Qt6 compatible”, otherwise some icons aren’t shown.

Thanks for the suggestion. That could have been an explanation if it wasn’t for the fact it only happens on one machine. I have not changed any theme from the defaults (that I am aware of).