"XXX is viewing" not showing on a form when multiple users are viewing the same record at the same time.
The reason this behavior is seen because the XXX is viewing functionality comes with the journal fields being a part of the Activity Formatter. The Additional Comments and Work Notes fields have to be added to the Activity Formatter to make that feature work.
In order to resolve this issue, navigate to any incident form, click on the Filter(funnel) icon next to the Activities in the Communication tab.
From the slush bucket that shows up, add the Additional Comments and Work Notes field from the available to the selected section and save this.
This will effectively combine the Comments, Work notes, and the Activities into a single field that work together. This will resolve the issue for you.