Hello,
Did you assign someone on your activity ?

As an assignment gives possibility to enter real work on the activity through Timesheets, activities with assignments cannot be in "Manual Progress" mode.

That's does matter.
Assignment is needed to note who is responsabile for a task and to show task for team member.
If a task whith assignment can't be manually followed, we lose one expected functionnality...

Assignment is not mandatory to know who is responsible for the activity as you have the field "Responsible" for this.
You can easily see who is responsible and a resource can see items he is responsible for (on today screen, with filter, ...)
Of course you can have one only responsible.

To point out that several resource can take part on one activity, assignment seems the best point of view.
And we tried (during devs) and allow manual progress on activities with assignments.
First issue is that automatically assigned resource can enter real work on the activity through timesheet.
So activity cannot be managed in "manual progress" any more. But this is not the major issue as we could automatically remove the "manual progress" mode on first entered real work.
The real issue is that if you have Assignement, you can define Assigned Work, Left Work and Planned Work (=Left work as long as no real work is entered).
And ProjeQtOr core automatically sums up Assigned data to the activity (assigned, left and planned).
So Real would be calculated from Validated x Progress, and left from assignment... so data would be incoherent.
At the opposit, if we don't sum up values from assignment, we may have data inconsistency between assignent and activity.
That(s why we reverted to what was planned on analysis of need to remove Manual Progress when an assignment exist on the activity.

You're right, it's a mystake.
responsabile is enought to restrict elements view and with no allocation, manual setup work fine.