I had a horse who injured her right hind leg & for years afterwards did not take the left lead though she appeared sound. I didn't force the issue until I saw her taking the left lead on her own but by then to her a left lead cue meant buck. She was very balanced on the wrong lead & for what she needed to do it didn't matter.
Maybe something like that happened to your horse?
Because you asked I will say that your riding is not very precise & your horse does not look set up for success in getting leads. I would suggest getting better hindquarter & shoulder control at slower speeds along with collection before asking for the unfavored lead.