Hey Alex,
I asked our tech artists this question, but as it usually is the case, as soon as you want to debug it, the issue doesn’t happen. I’ll post an update if we find out the answer to that question.
We have been trying to isolate a repro case without success as well. Our observed cases points to this having something to do with dependencies changing. E.g. a change to a material function used in the affected material. But no solid repro I’m afraid.
I think however, that the material preview issue described in UE-200553 is closely related, and that one has good repro steps. Would investigating that more closely be possible on Epic’s side?
Thanks,
Gábor