When he is wearing the bridle, he curls his lip, almost like he;s "flashing his grill". It's just on the left side, and it's not like he does it and then stops- he HOLDS it there. The muscle almost tenses up and just stays like that. I use a flash noseband and that seems to hold it down a little, but he still kind of does it underneath. I use just a regular D-ring snaffle, and I've tried other bits with him and he still does it. He did it with the last owner, too, and although she didn't say anything I can see it in one of his sale pictures. Anyone know why he might do this? It doesn't cause any behavioral problems, and it doesn't seem to bother him- it just puzzles me. I've never seen anything like it before.
Here is the sale picture where, if you look at his mouth, he's doing it.
*ahem* NOT looking for jump critiques, pleaseandthankyou,(we are both still learning) but here are a few more where you can see it:
And I am not pulling on his mouth. As you can see, my reins are often very giving, with a bit of slack.
But only on the left side. Not on the right:
It's not just riding, either. It's as soon as you put on the bridle to until you take it off.
So- have I stumped you all, or does someone know what this may be?