Outposts prefer not to spawn on the first island in spongeforge servers. They give the WARNING: OUTPOST NOT GENERATED ON AN ISLAND! when trying to generate. I have only encountered an outpost spawning on the main island once after testing a few times, which is a lot more than without spongeforge.

Spongeforge no longer exists in 1.16

This also happens without Spongeforge, but just less frequently.

This appears to be common with pre-generated islands mostly.