In desktop mode, comments added to a record are displayed in the activity stream, but in mobile mode, the comments aren't being added to the activity stream.
A field with the label Comments was added to the form in the mobile view, but it wasn't the same Comments field configured for the activity stream.
Remove the incorrect Comments field from the mobile view and add the correct Comments field.
1. Open any of the affected records.
2. Click the record’s context menu and select View > Mobile
3. Click the record’s context menu and select Configure > Form Layout
4. Remove the incorrect Comments field from the Selected slushbucket.
5. Add the correct Comments field to the Selected slushbucket.
6. Click Save.