Apps not appearing in Device Administrators
After thoroughly debugging this issue, I've found the cause. It seems to be a bug with how Adoptable Storage is implemented on my ROM. Any apps that are stored on external storage are not shown in the Device Administrators screen.

When using Titanium Backup, I came across the same issue as newly installed applications were not visible. 

As Android automatically manages storage, it installed all new apps on the External Storage which led to this issue. Applications marked as device administrators are forced onto the internal storage, meaning any updates they receive won't move them onto the internal storage

How to fix:


Open up Apps in Settings and navigate to the problematic application
Click on Storage and select Change
Select Internal Shared Storage and then confirm by pressing Move