ya thats normal. my TM 1911A1 could barely empty its 24 round mag with a single charge. but if you dont rapid fire your pistol it gets better. i find warming the mag in my hand prior to rapid firing the gun tends to be able to squeeze the last rounds out but slide might still not lock back.
today i bought some Trecel mags for my single stack 1911 and i was able to dry fire it 40 times before the gas runs out so im not sure if that mag holds more propane or dry firing uses less gas. but it seems to be an exact copy of my TM mag except 40 bangs VS 20 something bangs