Did some more experiments and found out, that these issues do not appear in the editor as frequently.
often, where Lumen breaks in the cooked game, it still works fine in editor.
I already removed trees because I thought they had an effect, but no.
Packaged game (on that location, Lumen works just fine):
…moving forward 1m, and suddenly it breaks:
In Editor: same location as in the “broken” image above, it works (some differences in light, because some stuff isnt enabled in the editor.):
Pretty much all meshes in there are Nanite (Except stuff that contains translucency/masked stuff), largest ones are 4x8m, smallest ones are like 10x10cm. (the colored walls are ISMs, the same goes for the randomly generated metal-walls, but still nanite meshes.)
Mesh Distance Fields:
Lumen Visualization (when it works):
Here is what MDF and Lumen look like when Lumen breaks:
So, MDFs are fine, Lumen not so much.
I also noticed, that I can make lumen generate the GI by just randomly clicking on meshes, and it works as long as I click on something. (As soon as I stop clicking, the GI gets slowly removed again.)
So, I would say that the Scene/meshes etc. are fine, but under certain conditions lumen just stops updating its stuff properly.
Video of what is described above:
This clearly shows that lumen is capable of generating GI for the scene just fine, but “decides” to not do so unless i constantly click around. I also dont see any other obvious mistakes on my end.
EDIT:
Restarted again, but now the same scene that was broken previously, does work like it should:
Now going to try if this location will break in the packaged game again.
EDIT: yes, partially breaks:
At this point, I am very sure that this is an issue with Lumen, since it shouldnt calculate different results in the same scene.
All I did was to package the game, I didnt touch anything - and thats enough to make it break.
Also, the other spot where it breaks moved down the hallway a bit, randomly, without me touching anything there at all.