It kinda sounds right to me anyway, such high elite people shouldnt convert unless they are of your same culture maybe
From a realistic perspective, most people have a price, if they aren't willing to fight for you, your price just isn't high enough. Maybe I can see noble guards (not a thing in the game, just made it up, unless you wanna refer to noble units) being really really loyal and won't join you if captured, but even the more well trained soldiers would join you for a price. Afterall, if you think about it, they are just fighting to get paid. It's not like the game actually have a culture morale system like Warband did, where you would lose morale for having troops of a faction that you are at war with. So they could add a cost to recruiting prisoners, and make it more expensive to recruit T5 and T6, but I honestly don't see why you can't recruit T5 and T6, from a logical perspective. They are just human and soldiers, trying to make a living, like most other people out there, they just happen to be better trained, maybe with some bigger ego/pride compared to other lower tiers.
Indeed you can't recruit t5 and above prisoners. And my ques is that it's for the balancing, so not going to get changed. Mods might help you there however as I expect it to be easy thing to mod.
Recruitable mod author has shown code snippet saying T5 and T6 prisoner recruitment is a bug from the conformity code block, and not an intentional feature (at least that's what he thought from the code not having a big enough parameter or something). Until TW says something about this being intentional, I'm going to go with it being a bug from the code, as we've seen one too many of those in the game at the moment.
I was using Tyni's bannerlord fix mod, which also addresses the T5 and T6 prisoner recruitment issue. I like it better than the recruitable mod, since Tyni's mod actually allow you to roll again on the same stack with a reduced probability, along with the mod having some other QOL improvements etc. Since i've seen a lot of people asking to have the game allow the conformity roll apply to multiple prisoners within a unit stack at once.