Can't view "Watchers" of a ticket after an update has been made to the ticket

Steps to recreate:
Visit a ticket
Make a change to the ticket (add text to a field)
Attempt to view "Watchers"

Expected:
List of "Watchers" is presented to user

Actual:
Nothing happens and "Watchers" event is not triggered when viewing dev console in Chrome

All users at my company are affected by this issue.
(v6.4.3#64018-sha1:4550402)