I had this once on a school horse- ended up being a pinched nerve, so horse presented lame when the rider was on the diagonal with her seat down when the shoulder was back.
Fixed with chiro adjustment wonderfully.
Similar to @mroades I had a horse in the past who was lame/sound if I changed my posting diagonal & he had an issue with his high hind suspensory…
And also, I have always considered different-feeling diagonals to be indicative of a problem in the hind end. I regularly ride my horses to feel the different diagonals (either in a figure-8 or on a straight line) to feel how they are using their hind legs.
Swapping diagonals (sometimes on a circle, sometimes on a straight line) are used to help locate the origin of lameness during lameness evals. It’s my experience that when they are noticeably different on one diagonal vs the other, that it is a lower-limb soft tissue injury versus bone or up high. I have seen horses with hind suspensories and also DDFT injuries be more lame on a specific diagonal.
I would call your vet and have them come out. Sometimes a feeling like that is an early sign of a soft tissue injury getting worse.
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