Locking cells unintentionally

I too have been unable to re-create this problem - but that does not mean that it cannot happen under certain circumstances or in a different version of Excel

One option would be a macro to auto-run whenever the values in required "unlocked" cells are amended
- this procedure unlocks required ranges any time any one of those cells is amended
Place in sheet module:

Const LockedCells = "C4:C8,F8:F12,I7:I11"
Private Sub Worksheet_Change(ByVal Target As Range)
    Dim r As Range: Set r = Range(LockedCells)
    If Not Intersect(r, Target) Is Nothing Then
        Me.Unprotect "password"
            r.Locked = False
        Me.Protect "password"
    End If
End Sub

It unlocks all the cells that should be unlocked

If you do not know the cause of the changes, you could do the same when any of the "unlocked" cells are selected