Question: If a follower catches imam at jalsa (sitting) position but imam stands up for third rakah before he can complete his tahiyyat, is he obliged to complete his tahiyyat and delay his standing even though he will have to repeat that jalsa anyway?
When you pray behind an imam, you are bound to follow him and utter the various remembrances (adhkar) and the like in the differing positions even if you join him late.
Reciting the tashahhud is mandatory (wajib) in the sitting position of the prayer, and reciting it takes precedence to following the imam.
Accordingly, you should finish it swiftly, and then stand to continue following him.
And Allah knows best.