While viewing any record (such as an incident) through the form widget (id=form), the Save button does not show up which is usually seen at the end of the form.
There is a UI action that is called through the widget to populate the Save button on the form.
When there is a UI Action Visibility set for the UI action for a specific view (such as the default view), the button becomes visible for only that view.
Now, the service portal opens up a record in the service portal view. Due to the above-mentioned UI Action visibility rule, the button does not appear on the service portal.
2. Check if there are Visibility already set for the UI Action for any other view and if so, create a visibility entry for the Service Portal view as well in order to allow the UI Action to appear on the portal. If there is a UI Action Visibility set to Exclude the Service Portal view, revert it.
Find more on UI Action Visibility here: