latest UE5 lumen reflection flicker only when playing

Hi!
When I have my lumen scene in the viewport, I get no flickering at all.


However, when I play the game, I get reflection flickering:

The red part flickers fast (in the screenshot I got a half transparent frame of the flickering part, it is normally flickering on or off very fast)

The flickering part is the part of the top light mesh that is NOT in screen space. My suspicion is that those get gathered over time and then reset, resulting in these flickerings.
(When I turn GI from Lumen to None or something else, the flickering also disappears, although I guess that has to do with something else)

I guess the viewport does not do proper lumen reflections already?

I would love some help, I have a deadline incoming and would like to fix that issue with some commands (no Postprocessing setting helped)

The flickering also heavily happens with glass (I guess because the flickering reflection is not in screen space sight)

Thank you very much!