The wither skeletons only respawn if you completely un render and then re render the nether fortress and even then, only 2/3 respawn
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[Mod] LateLag
Thank you for your report!
We're actually already tracking this issue in MCPE-21856, so I've resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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Bedrock has a LOT of issues with mob spawning/despawning, the logic is very different from the Java Edition. The only thing we can do to make them respawn easily is to change the game to Peaceful and then turn it back to Normal or Hard a few chunks away from the Fortress. :/