Repro Steps

1. Create a project with a multi-word title (ie. "COOL PROJECT" (because it's cool))
2. Create a milestone under that project. Name it anything you'd like ("Sprint 1" in this example)
3. Go to the "New Task" form.
4. In the "Tags" field, type Coo

Expected Result: The autocomplete list should contain both "COOL PROJECT" and "COOL PROJECT (Sprint 1)"
Actual Result: The autocomplete list only contains "COOL PROJECT"

However, if you begin typing the second word of your project's title, the milestone does appear in the results. This then gets cached on the front end, so subsequent attempts at typing in the first word do display the milestone. See the screenshots below if that's unclear:

First:

Second:

Third:

Another related issue: I don't think it's possible to get milestones to show up in the autocomplete if the parent project's title is only a single word.