For what it’s worth, we ultrasounded my late mares suspensories twice and didn’t see anything, and we xrayed her stifles and injected them twice and it didnt help. Nothing helped her, she’d go sounder if in really regular work but never perfect.
It might be a collateral ligament if it blocked low. My mare would lightswitch go from that humpy flat tire to nearly sound though, without any changes. Just… boom. So the standing for a second for the block could have “reset” whatever was ouchy. My mare was a weird, weird medical case though.