So the stoppage was always a 'failure to feed'(F2F) with the Sig mags? No double-feeds or failure to extract/eject?
Can you try a Sig mag solo - without the coupling set up? This is just to compare apples to apples, UNLESS your mil-spec mags that function properly were also coupled.
My buddy's new super hi-speed LWRC did the same thing. It jammed/F2F with the ultra hi-speed expensive mags like crazy, but when he tried my mil-spec 30-rounders... no issue. Performed flawlessly. The gunsmith later told my buddy the "gun and mags needed to get broken in," which makes little to no sense to me. I'm left wondering what the REAL reason is for the F2F's with a new gun/mag combo.
My only suggestion would be to experiment with those Sig mags. Try them individually. Try them fully loaded, then variations of partially loaded - and track your results. Which mag(s) jam, which don't, how many rounds before the malfunction etc.
Worst case scenario, keep the mil-spec mags handy.