This is a known issue with Gen 3 PMAGS due to the over insertion tab present on the back ridge of the mag. I did not know this was a "known issue" until it happened to me in February with a BCM blem lower I own. I did some research on the web and found more stories consistent to mine.
The Gen 3 PMAGS work just fine in my BCM EAG carbine, but I encountered the same issue you referenced above with my other BCM lower (28 rounds in mag). It is a blem lower, but I don't see that impacting the fit with the mag.
Before I searched the issue on the web, I got in touch with G&R tactical (where I purchased the lower) as well as BCM. G&R was the one who told me it was a "known issue" and BCM replied with the following:
Thank you for contacting us. The area of interference is not one which is tightly controlled by MIL SPEC dimensions. The reason is because the area has historically had no function and as a result no tight controls seen elsewhere on MIL SPEC parts. While some lower receivers may well function with protrusions in this area, others from the same lot may not. This is due to slight variances in forgings which is entirely permissible according to the MIL SPEC requirements. There are also a number of products on the market which may protrude into this area and reduce compatibility.
Please let us know if you have any additional questions.
I have a few of the Gen 3 mags, and didn't want to sell them and purchase Gen 2 or other mags. I ground down the insertion tab on one with a dremmel, and ran about 250 rounds through it along with multiple drop tests from 6' height onto concrete floors. I was worried grinding the insertion tab could weaken the structural integrity of the mag, but I haven't had any issues so far.