As said in the title aggresssive mobs spawn in mushroom island biome, which I believe should still be considered safe.

I actually feel like this happens by mistake as all overworld biomes are passed into mob spawn registry, so just excluding the "mushroom_island" and "mushroom_island_shore" should do the trick.

That is indeed a mistake. I'll push out a fix for it this week.

Should be fixed in the latest update.