Lo-Fi Games

What I can do to solve this issue?

Had the same problem - a workaround you could try: go with your squad to Tengu's Vault, save the game, import and immediately storm the vault. All unique prisoners were there in my case.

This probably happens due to the living world. I passed Burnt's tower once and the next time I went there, it seemed the spiders killed him because he was no longer there. I had kamikaze squads spawn even though the female shek leader (forgot name) was still alive (probably some other event triggered the squads that I had nothing to do with). Importing a save is the one way to fix these things as it resets the world back to the start.

Though to be honest I consider it kinda cool that the world moves regardless of your intervention and thus you can miss some things, making every "playthrough" unique. Imagine the modding possibilities for an even grander living world.

If is still a problem, they probably just died of starvation (because NPC's don't feed prisoners). Importing is the only available "fix".

They don't feed prisoners? Is it in general or just Tengu's Vault (the warden says something about eating their own shit if they want to eat).

Anyway maybe that should be posted as a separate bug report, but I can't confirm that myself as I've never noticed (been to Tengu's maybe thrice in total and it never had unique recruits, but didn't know there were supposed to be some). If you have noticed then you should post it separately (prisoners don't get fed by warden NPCs or something).