Thanks for the response. So I went through all the regular troubleshooting steps 1-8, but unfortunately it did not resolve the issue.
After digging a bit deeper, I've found the following.
- It is related to my child theme.
- I have a custom "_nav-primary.php" file - when I rename that, the mobile menu works perfectly.
I fixed the issue by replacing the content of my custom _nav-primary.php with the content found at the parent theme's file: /x/framework/legacy/cranium/headers/views/global/_nav-primary.php.
It looks like at some point the _nav-primary.php file was either deprecated or moved.
Either way, I've fixed my own issue. Thanks anyway!