I ended up 'solving' this by picking a static gradient art background for pre-footer sections, such that solid footer colors blend into them uniformly. So not really a solution so much as avoiding the problem, but it accomplishes what I was trying to do.
Appreciate the suggestion, @tuanphan! I did as you suggested, and stylized the section to match, and the exact same issue recurred. However, in replicating it in this manner, I think I've discovered the root cause: if, for a section, the horizontal gap width is set above ~35px or so, this problem occurs, and the severity is exaggerated the larger the gap with is set to.
Curious if anyone can replicate this, but that reads to me as a bug with whatever layout logic Squarespace uses. Will spend some time today to interrogate this further/understand the bug better.