Why doesn't the global or per pin assignment for the unused transceiver channel work for the Intel® Stratix®10 L- or H-Tile design when all the channels in a transceiver tile are unused?

In the Intel® Stratix®10 L- or H-Tile design, for the preserve unused channel QSF to take effect, at least one channel in a transceiver tile needs to be instantiated in the design. You can create a dummy channel in a Tile, and apply the global or per pin assignment.