Update 2021-04-09
Looks like this isn't an issue with MIMWAL after all (thankfully). Turns out, I had a race condition where a user would transition into a set twice, nearly simultaneously. Not something you'd expect, right? I suspect it had something to do with a nested set. Possibly even an issue with the version installed (4.4.1302.0). Anyway, I believe it's fixed after I flattened the transition set, meaning that I took the criteria of the nested set and added that directly to the parent set.
Unless you hear otherwise, you can assume that I had no more issues. :^)