I've experienced this a few times and while I would report this to Atlassian as a bug I cant find where I would do that. I just go in circles trying to report an issues.
When trying to organize my sprints in the backlog view I drag multiple tickets and try and drop them into a different sprint, but the tickets all disappear. They are removed from all sprints, including the backlog. I've found after a bunch of searching and discovered that their sprint has been set to NULL.
I don't really need community support on this, I need to report a bug. But because raising a bug is impossible this is the only thing I can do.
Making it impossible to raise bugs is a good strategy. You cant have bugs in your system if nobody can report them. BRAVO!