So far I've only seen this on Stack Overflow, which is probably due to its topbar design being different than most of the network, but in case there might be more sites impacted, I'm reporting it here on main Meta.
If I open a user profile on MSO today, this is what I see (Windows 10, Firefox 64.0); the
Stack Overflow sees one of its "bars" slightly hidden.
It used not to be the case, as evidenced by this archived version of the same profile:
Browsing MSE and MSO for
[bug] user profile and other variations did not yield an obvious duplicate.