We've just upgraded our live site and several users reporting seeing this happen. This is happening on Edge though, not iOS.
Our live site also uses a style by @Russ / @Steve F but given we saw it originally happen on the default style I assume it's not being caused by anything they've done and it's something in the XF core.
I would still try to rule out the style Rob, make a default style and let them test. I can say we do not do anything special in the nav though. Also might a be good idea to check the browser console for any errors.