Mobs spawning in mushroom island portals

Steps: play as usual, with mob spawning on. No mobs spawning in mushroom biome, as intended. Enter nether portal from overworld in said biome.

Expected result: no overworld mobs should be in the portal or surrounding the portal in the nether.

Actual result: assorted hostile mobs (creepers, zombie villagers, and phantoms) are in the nether. It is assumed they spawn in the portal frame or walk in, but should not be spawning due to the mushroom biome.