How does your support and development team handle this?
I’ve only needed to do this a few times and I think all but one was customer error/misunderstanding, not bug. If you haven’t already, send them a screen recording of how it should work (rather than just a text or voice explanation) and ask them to confirm that theirs is actually different.
It can be a bit of work, and it presupposes a certain understanding of the system in question, but one can absolutely do this. It's basically my day job.
Integrate better error logging and reporting systems like Sentry, LogRocket; both in backend and client side. This particular case gives me to hints to look for data specific to this user.