This follows on some discussion from T330792.
I believe the Bug: #nnn was used to refer to Bugzilla in the past - how are you planning to avoid confusion with that?
Or maybe we just shouldn't use the custom bug tracker integration to avoid confusion, remove the gitlab-phabricator matching for #nnn, and standardize on the Tnnn syntax, since that's what people are already used to. I'm not sure the integration adds all that much value.
My vote is for standardizing on Tnnn, as that is what I expect as a Phabricator user. It also avoids your ambiguity issue.
I think I agree, and this also cuts in half the amount of search traffic hitting our GitLab instance every time a Phabricator task is viewed, which is probably not a bad idea.
Bug: T324149 Bug: T330792