Pull requests are randomly automatically declined (BB-8227)

Hey Bitbucket,
We've noticed an issue where pull requests occasionally lose the commits they're associated with a message like this:

It seems that some or all of the commits that make up this pull request have been removed.

That's not true, though, and updating the pull request usually rectifies the issue. Occasionally, though, pull requests are declined by phantoms, leaving no trace as to why, just suddenly appearing in the declined list. What's the deal?