I've checked and rechecked this multiple times, but my client and I can't seem to make new submenu items appear on the mega menu, although they will appear in the Off-Canvas Menu.
In this case, the "Give Online" item is the culprit here. Also, I was able to duplicate this issue with a test menu item. Parent item is Giving.
![https://www.icloud.com/iclouddrive/0bX3j46VgGmZEHfpI6fYP2UrQ#bmpc-megamenu]