I have this exact issue, also running 1.17.3 with a handful of emails directed to their own queue. I updated from 1.15.8 straight to 1.17.3 and oauth last week, the issue was driving me crazy as some emails were directed to the right department but not all.
It being because of the alias now makes sense why I didn't pick up on it before was because the only other email with an alias is itsupport@ which is also the system default so never knew the alias for this was actually just hitting the default department.
I have just added the alias for this 'rouge' department, lets see what happens..
I have just seen there is a new update, so will need to install that this week!
thanks all