I would like to take on this issue

The problem seems to be in the _auto_legend_data function in legend.py. fill_between does not add anything that is considered in the calculation of "badness"
badness = (sum(legendBox.count_contains(line.vertices) for line in lines) + legendBox.count_contains(offsets) + legendBox.count_overlaps(bboxes) + sum(line.intersects_bbox(legendBox, filled=False) for line in lines))
I will try to resolve this until next Saturday.

FWIW, if you dig into this, there is also #23323 which appears to have a similar root cause. (Solving one of them is clearly enough to get a PR accepted, but it may be that one can quite easily solve both in one go.)

Okay, so the result of fill_between is a PolyCollection object. this is technically included in the badness calculation, but only kind of... as what it is actually including is the offset of each element in the PolyCollection. This is the case because a (path)collection is returned by scatter, where the offsets are the center points that should be considered (and also for other kinds of collections, like pcolor, etc)

Perhaps it could be treated as patch and line are? (Though wonder a bit about the performance impact especially for e.g. scatter, this is already a bit of a bottleneck that carries a warning if it takes too long.