Fast forward to now. Within the last month I'd say, he's taken to refusing to pick up his left lead - the one that used to be his good lead! I can't believe it would be from working his right side too much teaching him his right lead, because as soon as he became comfortable at picking it up, I started working both sides evenly again. He does prefer to bend right, but I can usually get him to bend left pretty well also, after a decent warm up.
He shows no signs of pain or discomfort that my trainer or I have noticed, and he is being ridden in the same tack he has always been ridden in. I thought perhaps it was me sitting crookedly or something, but I have lunged him and he still has a hard time picking it up.
When I talked to my trainer last night she said that recently a few of her friends had ridden him, and they are not quite as experienced as me. She thinks maybe that could have just tuned him down and now that he has the idea that he can get away with not taking the left lead, he just doesn't want to do it. He's a smart horse.
I always manage to get him to take it when we ride, it's just after much frustration and many many tries where he ends up taking the right lead instead.
Has anyone else ever dealt with this sort of thing?