Hello James,
Wow, that is helluva frustrating issue, thanks for reaching out to us!
I’ll do my best to help you fix that issue.
As a first, please make sure that your WordPress and the Modern Tribe plugins are up to date.
Do the tickets disappear from all events, or only some selected ones?
What I can think of is that you have some kind of a cleanup plugin that automatically erases the tickets every once in a while due to some misconfigured rule, so you check that.
If you would be willing to share your system information with me then I can likely help you further with finding the source of the issue.
Cheers,
Andras