An affected user is getting a message 'All activity hidden' in the 'Notes' section or other section which may contain the activity stream. The activity stream shows 0 activity.
This happens when the filter icon is used to deselect all configured activity. Using this filter corresponds to the user's "incident.activity.filter" preference which can be found in the [sys_user_preference] table.
The user should be able to use the filter icon on the right of the screen to add activity back onto the form. Please see the screenshot for reference.