I'm not sure why I haven't been able to find another thread about this.
This bug is nothing if not repeatable.
Issue being: the "tap F" resupply to grab new mags simply does not refill your magazines for beltfed machineguns unless they're completely empty.

Through some quick testing, this only seems to affect beltfed guns, standard and extended mags alike. (PKM, M240, MG3, M249)
I've very rarely had this issue with other guns as well, however the quickest workaround (except for doing a full "hold" resupply) is to shoot once and do a fast reload to ditch the mags you want to refill..... However there is no such reload option for beltfed magazines. And since a few months back, at least to my experience, this issue with refilling belt mags has been 100% repeatable.
This is not a visual / UI bug that would incorrectly display the amount of rounds left in a magazine, as attempting to refill, swapping to the magazines in question and then firing proves that the magazines did indeed not refill properly.