Alright thank you, I'll look into it
A friend I'm playing with has the theory that they aren't spawning because we haven't reached Gleba.
However, I'm looking at the in-game Factoriopedia and noticing that for some reason, the Stomper Spawners still require spores, and so they cannot spawn anything due to not consuming pollution.
If you have Discord, I can send you screenshots by DM.