As a side note, I have seen this pattern of too steep a boot leading to too much knee and waist flex and the bottom sticking out, Not quite a classic backseat problem, but in the same family. I don't know if that is what you are doing.bending at the knees too much and not enough ankle flexion. A steep angled boot could do that,
One of my Steep and Deep camps the whole group (including the instructor) went to the bootfitter one evening because two of the women students were having issues. (As I said, bootfitting is a spectator sport.) It drove me crazy that the instructor didn't really describe the problem and they were fiddling with side-to-side canting. I finally spoke up and at least got the fitter to measure her boots (eighteen degrees) but nobody took me seriously enough to do anything about it. Predictably, there was no improvment the next day. Sigh. [The other woman was helped a lot by her cant adjustment.]