Reported upstream: https://secure.phabricator.com/T6003 (Wontfix)
@Jalexander says: Is there a particular reason why Phabricator can't have the same username requirements as mediawiki so that people can easily have ACTUAL SUL and use the same name everywhere?
There might be other frequent cases, but we haven't identified them yet.
Phabricator lets you create an account with an alternative spelling (i.e. "Philippe-WMF" instead of "Philippe (WMF)"). If username accounts can be edited at a later stage (?) then a short term option is to ask users for some patience and imagination while we resolve this issue upstream in the best way technically possible.