I occasionally observe this too. (Both in Chrome and in Edge.) It’s actually fairly easy to make manifest on Windows when using the snipping tool. But I’ve not taken the time to make a 100% repeatable example (not sure there even is one).
I’m not sure this is 100% Bubble-specific browser behavior. I’m not sure that many folks HAVE noticed it.
The dotted frame seems to be a CSS feature appearing when an element is focussed. Got it with IE and Firefox on Windows 10, within a page where the user is not logged in yet. The dotted frame may show around buttons, text elements and icons, and according to anwarsby around groups. It really disturbs the aesthetics of the app while serving no purpose at all. Should be generally removed, it makes the app look unprofessional.