Mine does the same thing. The DPMS mag body hits the bolt-hold-open of the SFAR and jams up the works. Sample size of one SFAR and one DPMS mag.
Pull your lower off and stick the DPMS mag in the well, push down on the BHO, and you will see what I'm talking about.
I imagine you could cut the BHO back a hair on each side of the nub projection to make clearance, and the SFAR would run DPMS mags without a problem.
Tony
Edited to add - In my second pic, it appears there is clearance, but there is not. The mag hits the BHO and prevents it from rotating downward.