Help with that outside shoulder

I'm not sure "dropping the shoulder" is the correct term here. Basically when I reach the certain corner her outside shoulder "stick out" really badly. For some reason it happens in just one particular corner in one direction (other 3 corners we have no problems with). I don't think I'm "loosing" it really (as there is a contact on outside and I keep outside leg on). It mostly happens in beginning of ride and if I tap that shoulder with the whip she "sticks" it back in. But I'm wondering what I do wrong and how to correct it. Funny enough she does NOT exhibit this problem at the lesson in trainer's arena.

Is it the corner near the barn? My horse pops a shoulder when he's starting to get an attitude, and he often does it at the farthest point in the ring from the barn. Trainer says outside rein, outside leg. Try to anticipate and be strong with the opposite leg before you come to that corner.

Let go of your inside rein and sit in the middle of the horse. First, look to your own riding, then look to the horse.

Try counter bending him going into that corner. Null it out.

When this happens are you by chance going into a left turn corner and it feels like she is drifting to the right, even though she is bent to the left? Overbent, perhaps? If there was no fence there, would she drift outward toward the right? LIke a magnet is pulling her?

Sounds like her mind is going somewhere outside of the arena at that point. Next time she does it observe her ears and eyes and try to notice when her mind starts going that way, if it is well before the shoulder falling out happens.
If you can catch that happening, do something that will get her attention on you, like a change of gait or move her off the wall or stop or anything to interupt that thought in her. Then start again toward the corner. That's one way to shake things up.

You can push her shoulder back in with leg and outside rein, you can counterbend, or just to mess with her mind, do a volte ( a tiny circle in the corner there, going in the opposite dirctione she wants to go. So , she is falling shoulder out to the right, chances are you might be overbending to the inside (left) trying to go through that corner, she drops her shoulder, you just take her into a tight circle to the LEFT. This will disengage her from that drift to the right. Set it up and try again.

Kinda. It's the corner where she can see neighbor's barn/horses. And yes it does look like attitude (because she often pins ears too and tries to turn the nose in that direction)

Thank you All for suggestions!

Yes, I think her mind goes away in that corner (in very beginning, then she's all in work). I tried stronger outside leg/outside rein today and keep her attention on me more before we got to the corner, and it seemed to work.

Just a suggestion and some ppl might disagree on me with this one but i'll throw it out there anyway I was riding a reining stud not that long ago that liked to be right on the rail so when I would take him around his corners to do a run down he would drop his inside shoulder and pop his outside out to try and get closer to the rail, my trainer at the time said lift the inside rein and press with my outside leg at the girth to lift his rubs and push his shoulders back in place (if that makes sence)