After converting the world there are some chunks in the nether that surround the loaded area and can spawn overworld mobs. So far I've seen creepers, zombies, baby zombies, spiders (other players also have reported slimes to spawn in the nether).
Steps to reproduce
1 Create a new world on Minecraft Xbox one edition with the following setting and info
- build 1.12.1783.0 7.0
- seed: 12345
(rest of the settings are left to default)
2 Load the world and create a nether portal at 59,69,251
3 Enter the nether and wait a little while and rotate the camera to get all chunks loaded that surround the player.
4 Save and Exit
5 On Minecraft 126.96.36.199 (bedrock codebase) Import the world as usual.
6 Put the game on hard difficulty
7 Look around the edge of the area that was loaded in the previous version
8 Notice that overworld mobs spawn in certain chunks
9 If step 8 did not work run the kill entity command and retry 7/8
No overworld mobs to spawn in the nether