2nd subcategory navigation stopped functioning - Did Shopify discontinue backward compatibility?

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My store uses We Are Underground's Fashionopolism theme that is 5 years old. On Monday of this week (July 26th 2021) I noticed that my website stopped displaying 2nd level subcategories in my waterfall navigation menu. The website in question is www.soundingstone.com . As navigation drop downs open to reveal the 1st level subcategories, the second levels will not open to display on desktop with chrome or firefox browsers. To add to the mystery, the mobile view of my website works correctly showing both 1st and 2nd level subcategories.

I have made no recent theme code changes, I have backups of my theme dating back years and non of the backups function correctly either which leads me to believe that shopify has now stopped supporting backward compatibility for non-nested navigation driven websites. 

The two shopify support chats that I have done have been 60 minutes each with both gurus saying that its a theme issue and I should contact the theme provider for support. Having contacted the theme provider, they don't have have solution other than updating my theme and redoing all of my navigation, taking advantage of nested menu navigation. My website doesn't utilize nested menus. It was created 5 years ago when nested menu navigation was not available.   

All in all, it would take me a lot of hours formatting the new theme version, making code adjustments and having code for about 25 apps that I utilize in my store reinstalled. 

I've exhausted all avenues and can't find a solution on my own, anyone else have any bright ideas?