A third-party player's UUID is hashed from the source tag byte for byte,
so the tag is a permanent namespace: change it and every player of that
source comes back as someone new, with their playerdata, permissions,
account links and reclaim bans left behind. Nothing said so, and a tag
with a stray leading or trailing space, which nobody can see in the
file, loaded as a brand new namespace.
Such a tag is now rejected at load, and the AuthSourceConfig doc states
that the tag is permanent, case included. The charset stays otherwise
open: tightening it would force existing installs to rename, which is
the very thing that rekeys their players.
The new test loads a tag with a trailing space, a leading space and a
trailing tab through LoadNano; all three loaded before this change.