It seems like URL’s to user profiles are not case sensitive.
https://meta.discourse.org/u/DiscObOt works just as well as
But the canonical URL in the first variation is not canonicalized into the ‘official’ case of the username, possibly leading to duplicate content issues or the wrong variation being indexed.
So the canonical URL of the first variation is:
<link rel="canonical" href="https://meta.discourse.org/u/DiscObOt" />
while it should be
<link rel="canonical" href="https://meta.discourse.org/u/discobot" />
leading to the ‘correctly’ spelled variant.