Hey @Kobber,
I couldn’t replicate the issue in my site so this case could be unique to your site.
The Off Canvas Menu only shows up at first load and when you load in incognito or private browser mode. The Off Canvas should be hidden out of the box by Pro’s stylesheet. With that said, there is stylesheet loading delay in your site possibly due to your website optimization and/or server configuration.

Anyway, there’s only one way to find out if this is a theme issue and that is to copy your live site to staging and remove all optimizations, customizations and 3rd party plugins in the staging server. If the issue persists without all those, please give us WordPress admin and FTP access to your staging site and we’ll investigate.
By the way, we now have an optimization service called Boost. You might find it helpful if you get stuck with regards to website optimization.
Thanks.