I've noticed the same thing with a few guns. Mk.18, Uzi, and Alpha AK I've all seen do it.
Weird thing with the Mk.17 is the charging handle will lock to the rear as it should when the mag runs empty, then starting the reload will cause it to slowly slide forward.
I can't seem to reproduce it in local play, and I don't typically record my gaming. I'll see if I can catch it sometime.