Ok, actually the issue seems to be that QDateTime::toString() needs to be passed a format. I guess maybe without the format the second part of the hash returns a non-string value of some kind, because both parts of the hash work on their own, it’s only when concatenated that I get the bug.
Why that should only cause a problem on some specific systems I don’t know. Maybe it’s something to do with me using clang not gcc, no idea.
In any case I’ve fixed it, PR here.