Yeah, that doesnt sound like a good thing. Youre probably stretching / thinning out your lips and pressing the mouthpiece against them. I think it helps our endurance if we do the opposite, pushing the corners inward/forward towards the mouthpiece.

I havent encountered the smile (pulling the corners backward) too much, but have caught myself doing it occasionally since switching to a bigger mouthpiece (1.5 C) when I try to ascend and open up my aperture, get a better sound.

Its probably something youve done for a while. Awareness of it is probably half the solution.

Focusing on the air column and visualizing the aperture in your mind, tightening the corners inward horizontally from the sides inward (oooohh). Some time away from the horn thinking / doing that / visualizing it should help too.

And of course harmonic slurs changing pitch by engaging the corners inward towards the air column / center of the mouthpiece.

Just a matter of getting used to new sensations with better/proper form and experiencing the efficiency and resonance of the sound.

And of course watching the WindWorks videos and doing the course will help.

Hope that helps, good luck!