It has happened so many times now that I truly cannot fathom the explanation, especially since relatively simple flag activations manually with a mod have tended to cause her to work properly when Larian just cannot ever seem to take that final step to full proper functionality.
I'm curious, what mod is that? I have not tried to recruit Minthara yet because I've seen the bugged hell my friends who wanted to recruit and romance her have gone through- 3 months and unable to even trigger her kisses, Minthara acting like they've broken up with her or cheated on her, her dialogue sometimes simply not loading at all.
I know of the "daughter of Lolth" mod. This will give you Minthara as companion in act 2 if you knock her out in the goblin camp. Without all the complications I 've read here concerning temporary hostile status etc..... Just a knockout does the trick. I played this, but, very soon after I got her company, I stopped , deleted the mod and restarted to do the "evil route". The dialogues with her in the modded game didn't make sense because the timeline was screwed up. She talked about events that had not happened yet and so on....
I went back to Nexus to verify if the name of the mod was correct and there's a warning now that the mod is no longer needed after patch 5.
I played a (good -- well, mostly) dark urge run on the "evil" Minthara route. It was a very interesting game, but Minthara romance was as good as unexisting until the end of the game. I don't know if it is different if you do submit to Bhaal, but I don't expect it. I have a save just before deciding to accept or reject the ascension to Bhaal's chosen. But I'm not very motivated to restart from there because I don't really expect a difference , as the Minthara dialogues just seem incomplete, not yet developed, under construction.