If anyone is still running into this… We ran into the same thing and running garbage collection a bit more aggressively (i.e. manually running it every 12 seconds right now) SEEMS to have worked around it.
GetWorld()->ForceGarbageCollection(true);
The issue seemed to be memory-related, so felt like that made sense to try. Not sure how much sense that makes but so far it seems to help.
In our case the issue seemed to be triggered by using K2_DrawMaterial(...).