We do have some custom CSS, but it’s pretty basic and shouldn’t affect this. Nonetheless, I disabled our custom theme using safe-mode and got the same problem.
After some more tests, I noticed that this only gets triggered depending on the available space. Changing the available viewport width, it works fine:
Or, if tags are enabled in Discourse, it’s also fine:
But, if there’s a wide space for the category field (and if tags are not enabled), then it can overlap the preview.