That's just how Omega Red works, you can't add any more than 10 unless death field is active
Maybe you used him with red hulk and got confused that it like that without the synergy?
I can seem to put more than 10 spore on the opponent without activating death field. Is this happening to anyone?
Ironically He actually is bugged though. With death spores active, even one death spore should be applying damage but randomly in fight it only works at 10+ even with death field