Hey @stefanvandervlag,
I saved your Salespage Preview page as a template and loaded it to my test site and did what you’ve shown in your video. The result was, the issue does not happen on my end. This leads me to conclude that this is not an issue with Cornerstone but rather some limitation or conflict with your server and/or WordPress setup.
In terms of server limitations though, you have greater limits than my test site so this might not be the cause. The only significant difference with my test site and yours is this:
Your setup is highlighted in red.

Basing on the screenshot, this could be a database limitation.
Regretfully, we are not the right personnel to troubleshoot or correct server setups so I’d recommend that you try a different host.
The other areas you also need to look at are third party plugins especially optimization plugins. I saw you’re using WP Rocket. Please clear all its caches and deactivate it and all other plugins then check if the issue persists. If it still does persist, you will need to proceed with testing other hosts.
Thanks.