The Deutsche Bahn interface does not automatically recognize when a traveler using a BahnCard 100 requires only a ticket - or - a seat reservation.
Therefore it is necessary to select "Seat only (no ticket)" specifically should only a seat be required.
Available free seats are only offered - via - a previous registration with BahnAccount.
No price information is offered when selecting Bahn-Business BahnCard100.
Therefore, the option "Seat only (no ticket)" must be specified and selected.