After some consultation it turns out that Jaheira being in Star: Archer form for druids was the cause. Still a bug, but at least I can work around it now.
Huh... so they haven't fixed that behavior yet... bummer.
I've had this happen with both Halsin (during the Oliver encounter in Act II) and Jaheira in Act III.
It doesn't seem to be a problem in *some* dialogue sequences - Jaheira for example would act normally while in Starry Form during the first conversation with Nine Fingers (where the party is asking for information about Minsc) - but then be AWOL during the conversation with Uktar (about Minsc/vault keys) right after.
I'm currently running a Star Druid Tav and haven't had any issues with Starry Form - seems to only affect companion Star Druids who appear to be treated as if they were in actual Wildshape (meaning: transformed into an animal) during most conversations and thus will be "missing" from those. And I don't think this is intended behavior for Starry Form as you can interact normally with people as a "Starry Form"-Tav and Tav/Durge can also start a conversation with a companion Druid while said companion is in Starry Form. Pretty sure neither of the two would work with "true" Wildshapes.