FWIW if anyone cares - I tried a different spacebar cap and it's returning normally.
Weighing the one that returns ok I get a weight of 6.84g, whereas the other one I had on was 8.99g. I don't know exactly what the threshold is, but somewhere between those weights the switch is unable to return the key consistently (it would return if I quickly removed the weight and let it spring up, but if you slowly lifted up on the key it would stick on the click bar).