Weird, but also not so, depending on what load you are using.
I can stuff 20-22 rounds of 200-220gr subs in a G2 30rd Pmag without issues. It gets dicey after that. Skinny supers? (That said, there is an acknowledged issue with standard PMags and some 300BLK rounds)
20round Pmags, no issue.
Lancer Mags, full load. 20 or 30 rounds mags. No issues.
If I remember correctly the modifications that Magpul made related to the little ribs on both sides at the top of the mag, that corresponds to where the shoulder would be on a 223/5.56 (as well as thinning down the central rib thickness), which would not be in the same place on a fat 300blk round.
I think there is a video out there somewhere on modifying older PMags to 'fix' the issue. If you've got extra PMags you could try the modification on one. I'll see if I can find the vid.
As a side note, while 300BLK Pmags are no where near as cheap (on sale) as you can find regular PMags, Primary Arms does get them down to $10.49-$11.99 on sale now and then.