The really interesting thing about riding position is that quite often the visible “flaw” is not the cause but rather the result of something else.
I have a good friend who is a capable, brave, physically fit, experienced rider, with a fair training tool box, a sticky seat, and a calming effect on hot horses. She’s actually all around a good rider. But she has perennial trouble with her leg going backwards when she uses it, and not keeping her heel down, and sometimes tipping forward. It messes with the picture she presents and sometimes makes her look like a much less competent rider than she really is.
Her problem is very mild scoliosis leading to some hip imbalance and some surprising limits on how she can move her leg in the saddle. She was only diagnosed with the scoliosis in her late 30s though it’s clearly been a lifetime thing that she’s always compensated for. Watching her try to work through this with chiro, seat lessons, and saddle fit has driven home to me how we don’t always know what’s going on with our own bodies let alone other people’s bodies, that gets in the way of good form in the saddle. She has made improvements in the past 5 years and those improvements have made her horse go better for sure. But simply shouting heels down! at her does nothing to fix it.
Anyhow I think quite often persistent visible errors in rider form are actually caused by problems in the torso core, which the rider might not even know about.