Yes, this is a known problem with our implementation that we will be fixing in the future.
Workaround for now: insert a non-breaking space (essentially a space) before or after the text depending on alignment and where you want the margin. You do this by entering Alt+0160 which is hold the Alt key and enter 0 then 1 then 6 then 0 on the keyboard and let up the alt key. This works in Chrome and most likely all browsers. Sorry about this inconvenience we are working on the design now for even better looking buttons, the best buttons! Thank you.