Nothing about that sounds like a problem I've heard reported before, so it may indeed have been a sort of momentary glitch that occurred because the game was "busy" doing something else. If you can replay that sequence with an archive save, it might help.
You're right - I re-played from a save taken just before I left the Planar Prison,and Jan didn't start the 'invented swarmer mates" banter untill after we had arrived back in the Five Flagons. Now I just need to use Shadowkeeper to find the right variables to set
Edit: Fixed now - all I need to do now is sort out the Jaheira dialogue problem - which is probably
not a banter pack issue.
Further Edit (6/10/04): What I did was lower his level in Shadowkeeper (leaving XP unaltered) then, when I loaded the game, he re-did the Banter and gained the ability.
Thanks
Charles