I could never get this working in these versions of the products. I worked around it by using includes and manual menus. Eventually, I dumped both Wagtail and django templating completely. Not because they weren't functional. This was just costing too much time and it didn't seem solvable. The bigger issue altogether seemed to be that CSS styling was too much of a hassle for that level of work. Two weeks later I switched to react/javascript on the front-end and turned django into a headless RestAPI using django-restframework. Django was not the issue, but I just didn't seem to be able to make any headway on getting themes and menus to behave the way I wanted using the combo I was using. Moving to Google @mui components made the entire thing do-able, simply because I could dump CSS altogether and code to what I wanted. Although I could've spent 6 months learning CSS and fixing the issues, that didn't seem to be a productive use of time. With Javascript I can still make my pages easily editable through config files and not have to worry about how they're displaying and clobbering everything around them.