I LOVE this mod! I removed the biters from my railworld and it worked great... But when I expand my rail network and explore new chunks biters spawn as normal. I have to disable the mod, load the game save it, enable the mod and load again for them to go away? Is this as expected?

Not at all, the mod is meant to also disable spawning, but does so in a way that other mods could conflict with if they mess around with biter spawning. This is somewhat on purpose, not only I can't predict all the myriad ways other mods can change biter spawning, trying to account for that could end up with me deconstructing stuff that aren't aggressive biters.

To not go too much in detail, either biter bases are being spawned manually, on a different map setting than "Enemy bases", or outside the enemy force.

I'd need to know what other mods you're running to make sure which of these three, or an unexpected bug (hah, pun), is behind this and see if a workaround is both possible and safe. Sorry and thank you >_<.

Having the same issue on my bob's mod playthrough. The only mod that is manipulating the map is RSO, so it's affecting biter spawns. I had an invasion of biters beyond the fog of war attack my stone mining outpost, I haven't revealed the area north of the stone patch in an effort to keep my save file small.

I have RSO running as well. Just a few QoL mods and the More Trains mod.

Sorry about the delay, but it's "fixed" in 2.0.0. RSO has a separate spawning engine than vanilla, so my mod can't exactly disable them, you need to go to the RSO settings and disable it there. The mod now warns when this is necessary and gives instructions on how to do it.