Hi there,
Some elements preview will be disabled if it causes conflict, some 3rd party plugins are harder to control and manage even if they are bundled, especially that they too change and update their standard which will occasionally cause conflict. Even if we try to 100% make it compatible, there will be cases that are out of our control. So yes, an immediate solution is to disable the preview for that particular element.
And separator will only work on the solid background color, and not with images. Then, change its color the same as the color of the next section.
As for borders, you may ignore it for now. The border works, but there are spaces between columns that separate them which is only visible in builder preview.
Thanks!