Late to this thread but chiming in because I encountered this evasion with my horse after he injured his shoulder. Specifically, he broke his scapula and did who knows what fascia and muscle damage (but thankfully no to minimal apparent nerve damage).
He also has trouble bringing that front leg up and out to the side. SI/HI and leg yield all work pretty much as normal. This horse had no problems with HP this direction prior to the injury. If anything it was the better side and the other side might get some head tilt and not enough bend.
Now he’s like, it would be so much easier to canter, even canter in place, than half pass right. You need to work on that front leg mobility and independence.
However, if you also can’t get a good HI, then that means your driving leg for the half pass is not strong and doesn’t want to carry enough weight. This would be my issue with my young horse who also collapses on the right shoulder, but he’s not ready at all for HP…we are still working on basic leg yield and SI without losing forward. If it is a basic strength and balance issue, then it is too early for HP until the other movements are easier and fluid. I do work on the idea of what he needs to with the hind legs as well as opening up that inside leg with the baby horse with walk pirouettes.
So do not forget to work that left hind leg. He has to carry weight on it as well as push in order to get the RF lighter. It’s probably also why he doesn’t want to stretch the ribs on the left side and give you more bend.
I would do a lot of leg yields with bend to work on this. Leg yields both ways. Then to SI. Both have to be able to be ridden pretty forward in trot because if he wants to get tight with the body that’s where your unintentional canter comes from. Once you have that feeling then you can probably go from SI to HP back to SI. Or leg yield towards the wall then HP away. Then try HP away and as soon as you might lose it, half halt then leg yield back, keeping some energy. Try to be loose/relaxed/minimal with your body. Light enough in the seat the hips can move. This is hard for me because my horse is more a push type but pushing or trying to block the evasions here doesn’t work…just makes him tight and he thinks canter is easier. Those exercises have given my guy more confidence to try the HP again and don’t get too ambitious with how many steps you do in the beginning because he may fall back out of balance and then try to fix that again with canter.