All our stories have disappeared from the sprints in backlog view, except the backlog itself.
If you drag a story up to a sprint it adds it.

Note the story now in the sprint above the backlog.
Then if you refresh the browser it does this:

Completely gone according to the backlog view. If I open the story its still in the sprint. If I view active sprints its fine. If I view issues and apply a filter to show my sprint its all good.
Sometimes a refresh brings everything back in the sprints. But its patchy and can sometimes disappear again before your eyes, so its not a filter somewhere. All of us are seeing the same behaviour so its not my browser / my computer.