Confirmed and reproduced. I would actually categorise this as a "bug" - it's quite likely to be UX-breaking for an inexperienced user.
Also, Cmd + U doesn't work on the new tab page to untoggle it at minimum. Should focus mode be disabled upon opening the new tab page, or is that too invasive? I don't use it, so not sure what makes sense... In any case, this is the sort of thing that could get a user pretty confused!