It happens on a page with nothing but the diagram code on it, not just when embedded in a page with other HTML, so it doesn't seem to be dependent on something being to the right or left of the diagram. The board itself looks the same with either betza.js or betzaNew.js, except for the gap, and on an iPhone (for both scripts) a square board will be squeezed so that the squares are vertically rectangular, as well as the board. I just tried it in Firefox of MacOS and there was no gap, so looks to be a Safari (Webkit) specific thing.
It happens on a page with nothing but the diagram code on it, not just when embedded in a page with other HTML, so it doesn't seem to be dependent on something being to the right or left of the diagram. The board itself looks the same with either betza.js or betzaNew.js, except for the gap, and on an iPhone (for both scripts) a square board will be squeezed so that the squares are vertically rectangular, as well as the board. I just tried it in Firefox of MacOS and there was no gap, so looks to be a Safari (Webkit) specific thing.