The Field Watcher is a very useful tool to have when troubleshooting field behaviour. It can show what changes have been applied to a field as well as what caused the change. To use the Field Watcher, right-click on the filed label and select 'Watch - [field_name]'
When using UI 16, you may notice that the Field Watcher is not available for the out of box Journal fields 'comments' and 'work_notes'. Right-click on the field label and you will not see the Context Menu provided by ServiceNow. You will see the regular right-click menu for your OS.
This is due to the Activity Formatter used in UI 16. The formatter will 'bundle' together Comment and Worknotes with the Activity Stream in order to render the newer interface which introduces features such as real-time updates, activity filtering, and user presence.
If you are running into issues with behavior changes for the journal fields, Field Watcher can show what scripts are making changes. You just have to remove the Activity Stream from the form (or move it to a different section) in order to stop the Activity Formatter from overriding the journal fields.
Right-click on the form header and select Configure > Form Layout
Find the section where the 'Activities (filtered)' field is on and move it to the 'Available list'
Save your changes and you should be redirected back to the form. Right-click on the Journal field you're working with and the ServiceNow Context Menu should now appear, along with the option to 'Watch - [field_name]'
All (when using UI 16)