I’m wondering if maybe she has the beginnings of navicular where she is sound for light riding, etc. but can’t handle circles like lunging without lameness. Or possibly arthritis. My first bet though would be navicular. If that’s the case she should be fine for a child’s light riding and trails, just not hard pounding work or lunging.
She does have some arthritis. I could see it in her hocks. Navicular wouldn’t surprise me.
When she went lame, it seemed to be very acute on one side, sharper than an arthritis symptom. In addition to maybe working her a bit hard for a first time out, she either bruised or pulled something.
And you’re right. From here on out shouldn’t really need to be lunged.