Clicking "Reset" on the Panels from Administration doesn't always restore the saved values

Steps to reproduce

1. Login as Admin
2. Go to 'Administer Wiki' > 'Look & Feel' > 'Panels'
3. Observe the value on "PANELS DISPLAYED ON THE RIGHT" it should be the factory default: "Help.TipsPanel.WebHome,Panels.MyRecentModifications,Help.SupportPanel.WebHome"
4. Click "Reset" button
5. Observe again the value on "PANELS DISPLAYED ON THE RIGHT"
6. On "PANELS DISPLAYED ON THE LEFT", add 'A.B'
7. Click 'Save', then refresh the page
8. Remove 'A.B'
9. Click "Reset" button
10. Observe the value on "PANELS DISPLAYED ON THE LEFT"

Expected results

Step 5: Nothing happened, as the "PANELS DISPLAYED ON THE RIGHT" value was not altered before hitting the "Reset" button.

Step 9: The panel/value 'A.B' is added back.

Actual results

Step 5: The last panel, "Help.SupportPanel.WebHome" gets removed from the field (see screenshots).

Step 9: The panel/value 'A.B' is not added back (thanks vmassol for reproducing it).

Tested and reproduced also on XWiki 14.10.21 (but the issue may be way older than that) and 16.2.0.

I don't know for sure if this should have been split in 2 tickets (if the cause it's not the same), but please let me know if it's the case.