When I'm placing roofs (the flat ones) with bgrade set to stone and place quickly, it starts having gaps where it seems the building animation didn't finish/bugged out in some places and you have cracks you can see through in your buildings (it's different for every player). Happens with foundations and flat roofs so far.

Yep same problem but with walls and floors. I've been waiting patiently for a fix but as there isn't one yet, I decided to make my voice heard here. For now, I've told players to do /bgrade 0 until it's sorted and then use /up (Building Grades plugin) to upgrade their base. Here's a screenshot of the issue.

I have the same problem and it drives me crazy, I just stopped using it all together.