Hi Oliver,
The focus outline is there for accessibility reasons. It does not prevent you from clicking, but highlights the active element. As Martin already mentioned, you can also deactivate the outline setting in the theme styles. All in all, this is not a bug.
Certainly not by default. This must be a specific problem with your setup, possibly a ::before or ::after pseudo-element that overlays everything else - but this has nothing to do with the focus outline, which is nothing more than an outline.
Try to reproduce the problem again in a fresh installation - I don’t think you will succeed, as I cannot see a general problem.