No, it happens randomly.
A user (who has previously had a working Folding postbit), no longer has it in their posts.
However, in the Folding admin tool, there are two reasons:
1) It shows the users postbit as "approved" when you search for him, but you can't find him in the "approved" list manually.
2) In other cases it shows the postbit as "pending" (as though their entry had been reset).
For postbits to work when approved, and to not reset.
Originally, if a user changed their Accounts details, their postbit entry would reset. We did sort that out in the sandbox stage though, so if the name wasn't changed, the postbit entry wouldn't change either. It's possible that might reset (in the second, "pending" case above).
No idea about why the postbit just wouldn't show when the tool says it should.
This users is listed in the admin tool as having a working postbit, but his profile clearly doesn't show it.