Hey thanks for the replies.
It's either # 5 (teres major) or 6 (teres minor)
It happens during the lift and relaxes afterward. It doesn't seize per se, just super tired and begins to cramp. On front raises, overhead, and bench is always happens at the lower portion of the movement.
I had some stability issues on my left shoulder and had surgery. Everything is good now but this Teres issues is present on both shoulders so it's either an anatomical issue (unlikely) or a muscle imbalance. Whatever it is, it's severely limited my shoulder development!
I've started doing more direct RC work and rear delts.