The bug

Zombie sieges can still spawn on mushroom islands, if the player triggering the siege is not in the mushroom fields biome.

To reproduce

1. Download and open this world: https://www.minecraftworldmap.com/worlds/_H5V5
2. Go next to one of the villagers in the ocean biome "lake"
3. Run /time set 17999 repeatedly
   → At some point, zombies will spawn on the mushroom island

Video

https://youtu.be/j00LtZgS6SM

Original description

I've constructed a village in a Mushroom Biome, it's currently populated by 7 villagers and a matching amount of beds. The Biome is an independent Mushroom island however there is an ocean Biome spot sort of in the middle of the island, it has got a diameter of roughly 30 blocks, which I have covered with grass blocks. After covering the area I made sure that the spot was well lit to prevent any hostile mobs to spawn (I also lit up any cave systems below it). 

A while later I noticed a siege happening, I witnessed it spawn roughly 20 blocks away, it did spawn in the Mushroom Fields Biome. The same thing happened the night after, where the siege spawned right next to the one before. The third night the siege spawned right on top of the village center (i.e. the bell). 

If I understand the Minecraft Wiki, a siege shouldn't spawn in a village populated by anything less than 20, also Mushroom Fields (and Shore) shouldn't be spawnable (with sieges). However the Wiki also says that the spawn of a siege ignores any light level, but I've heard that it was a bug that got resolved in 1.14.4 and that they now won't spawn as long as the area has a light level equal to or greater than 7.