What theme are you using? Are you using a special menu plugin like UberMenu?
We’re using a theme called Tyler and no special menu plug-in
The theme’s demo looks fine, so it’s not a browser issue.
Have you altered any of the theme’s code?
I’m working as part of a team so I’m not sure if someone has. I know we have WPBakery Visual Composer activated so that could have caused the issue? Oooh would it be possible to display the mobile site (rather than the laptop/desktop version) and fix the issue visually?
Every method I’ve tried to display the mobile view on a computer doesn’t show the issue.
Try deactivating WPBakery Visual Composer and see if that fixes it. Actually, try that with all your plugins. Backup first!
If that doesn’t work, consult the team to see if anyone’s modified the theme’s code.