Turns out this was due to our Topic type also having a field called subtopics which were also of [Topic!] type, but we didn’t ask for all the same fields (Actually just missing one of the subtopic fields in the main Topic query.)
The fix was simply to add that field to our main topic and we were good to go.
BUT this issue was a pain to debug, is there a good way of approaching these kinds of issues?
Or even a way to test for them to prevent them from happening?
Maybe log a warning if something is cached, but misses fields or something along those lines?