notifications don't work if assign/subscribe field is autocomplete

If the number of users is sufficiently high that the assign and subscribe fields are autocomplete, email notifications do not work correctly. Noticed this first on a site I administer, but can duplicate w/ a new Drupal 7 install (on Pantheon, if that matters).

Started with Drupal 7 install and 5 users; using only core modules except Devel and Entity API. Email notifications work fine:

1. User w/ role of Staff creates a ticket; notices sent to Staff and Admin, who is auto-subscribed to all tickets.
2. Admin assigns ticket to Manager; notices sent to Admin, Manager and Staff
3. Manager responds to the ticket; notices sent to Admin, Manager and Staff

Generated dummy users using the Devel module, bringing total number of users to 35. Assign and Subscribe fields now autocomplete rather than drop-down or checkboxes.

1. Staff creates ticket; notices sent to Admin and Staff (fine)
2. Admin assigns ticket to Manager; notices sent to Admin, Manager and Staff (fine)
3. Manager responds to ticket; notices sent to Admin and Manager (! no notice sent to Staff)
4. Manager responds again, this time manually adding Staff to Subscribed list; notice sent only to Manager

If I then remove 20 users, bringing the total down to 15, the Assign field is now a drop-down and the Subscribed list checkboxes, and the notifications again work just fine.