I have conducted a little more research and I think the FreeDebugCamra simply is not meant to be used by anyone without the internal build *maybe* because the "PluginDeveloper" is not included with the release build. The instance to it from the plugin manager is NULL. Calling the freecam gives a nullptr error to this, that's why it might be linked.
It previously worked because I could spawn a camera object and have a separate instance, but this does not seem to be possible anymore. And the singleton instance of FreeDebugCamera.GetInstance() does not comply with what you want it to do on the public build. I can not use it on stable atm either, so it must have been like that all the time.

This has been confirmed not working from me and mov3ax independently. But sure I can try the next exp update and see if it works there.

Thank you for the info Geez <3

I got some information that *maybe* this resolves the issue / has something to do with it: https://www.techpowerup.com/download/visual-c-redistributable-runtime-package-all-in-one/

After today's EXP patch the flickering is not as bad as it was before, but still present. It gets worse when you reach around 11 light sources though.