Hi Jason,
I’ve noticed a similar thing. I’ve been working up Christmas carols recently and am spending a lot of time at the second octave G and below. Nice sounds are had, but when I get back to ITM and try to go above the second A it doesn’t work well. Until I go back and remind my lips how to play those high notes again by doing octave jumps for a while, preferably at low volume. I just figure it’s muscle memory and the muscles need reminding. I don’t really think too hard about what my lips do, I just let them figure it out.
I also notice that a relaxed embouchure works way better for me than a tight one. Sometimes trying for the higher notes makes me pinch too much and I do have to catch myself for a moment and relax again.
YMMV
Carey