Alternate gear extension is not checked inflight by aircrews, except for testflights. In any case it has no effect on the brakes.
The alternate brake check is to be performed at regular intervals and came into force after problems on A320's which have the same alternate brake system design.
It is to be done at the end of a flight and only into a maintenance station.
Part of the check is to verify no redsidual pressure remains in the alternate brake system. Residual pressure can be removed by additional brake applications.
But I guess nobody checked. AKA no FE onboard.