Part1
Repro Steps:
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This issue is re-produced in OOB PSA Version3.x
1. Go to UCI app Project Service -> Project ->open a Project->go to 'Schedule' tab
2. Click 'Add Column' field on the view, there are 2 fields in the drop down list -'Category' and 'Description' , click on them, these fields will be added to the view.
3. Close this project record and open it again, go to 'Schedule' tab
Expected Results: 'Category' and 'Description' display on the view (fixed on the view)
Actual Results: 'Category' and 'Description' are not on the view.

Part2:
When working on above issue, I found another issue:
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I tried adding fields to the 'AdditionalColumns' view of Project task entity.
Expected Results: all fields should appear as a column in 'Schedule' tab view on Project form
Actual Results: some fields will show up in the 'Schedule' tab view but some not (e.g. 'Category', 'Description' and many other fields), it seems this does not support all data types.