I was trying to work around the issue temporarily by making a row that is only available on large and medium devices and checked the preview I noticed that nothing showed up for the medium preview. So I thought about the fact that I changed the settings for my breakpoints in the beaver builder general settings from the default 992px to 1150px for medium and 768px to 780px for small. I changed them back and the issue was gone. I then slowly upped it back from 992px to 1150px and everything was ok. Then I changed the small setting to 780px and everything was broken again.
So my problem is solved as I can live with the 768px instead of 780px for small, but this does seem like a bug to me. Does anyone have an idea why this is happening?