Thanks for all the reports everyone. It seems to be limited to a specific user case and I'm unable to reproduce it.
In other news, I know we've talked about this in the past but I've installed a private test forum with the latest XenForo software version from scratch. I had some issues upgrading our existing forum since it's so huge so for now it's just an empty forum where I test the functions. But I didn't deep dive the error logs, I'm sure I can upgrade it when I sit down with it for a bit.
The reason we're aiming for the latest software is of course security and performance but also that some of the add-ons we use are still being updated and maintained but require a newer forum software version than what we use so we're stuck with the little outdated functions. As an example we have an add-on for auto embedding of social media posts and content, ours is a little outdated so won't support all links.
Anyway, the new forum software is very basic and boring and I don't really like it but I'm sure I can match the feeling, style and features of our current forum after some work. I'll let you know when you can try it.