I wonder if this fix needed to be applied at a second location too for dealing with the composer preview?
https://github.com/discourse/discourse/commit/833af461ab5d9e170306172b68541bd62b09201a
It stemmed from
https://meta.discourse.org/t/group-mentions-that-begin-with-the-same-characters-may-be-incorrect/39892
Which had an issue with group names with dahses in them.