Mine yawns on occasion (and he clearly tries to hide them), but has never seemed like he was falling asleep. (Hm, maybe that's related to his drinking coffee all day?) It would bother me if he seemed sleepy fairly often.
If it was a single session with your T, I'd chalk it up to maybe he slept poorly the night before or is under the weather. But if it's happened multiple times recently (or at least in the past 6 months), then it would bother me. I do wonder if it could be the time of the session, as mentioned above. I'd consider bringing it up to him.
Regarding teletherapy, my T has said it's more difficult to focus in general, though he's gotten more used to it. So the teletherapy could play a role, too.