Yeah, I had seen this kind of bugs, too in BR, for a while.
However, this is really tricky to trigger it, everytime.
I mean, by switching teams, it did not trigger.
I just played a bit with bots, with WQ3 b2.1testpatch01, but I still do not really know in what case it happens :/
When displaying the scoreboard, it sure happens that scores drop to 0, but a few seconds after, the scoreboard shows the right number ... strange.
Of course, by spectating, you sure lose your score
... but that is not an issue.
Hmm, some more detective work is needed, though