In my playthrough ...


Jaheira didn't join my party for the Z'rell fight, though she and her Harpers were there in the combat they were all under AI control.
After the fight, I spoke to her, and established she'd like to join the fight with Ketheric, at which point she joined as a kind of summon (ie attached to my PC rather than a full party member).
But then I went down to the basement, and she said Ketheric wouldn't fall on his own sword and would rejoin when I was ready to fight.
After I'd cleared the basement (I think, though I was hoping to see Florick down there and didn't!) I went back up and spoke to her and she reattached herself to my party.
After the fight with Ketheric on the rooftop, she detached from my party and sent me after him. I was able to ask her if she wanted to come and she said yes, but that I already had enough people. I'm assuming that this is the point at which she'd actually join as a full party member, but I didn't dismiss anyone so went down the tower and finished off Ketheric without her.
I didn't long rest between starting the assault on Moonrise Towers and defeating Ketheric on the rooftop.

I hope it's helpful to compare that with your own game.

"You may call it 'nonsense' if you like, but I've heard nonsense, compared with which that would be as sensible as a dictionary!"