you are pushing the pump head on too far.. I have a Silca floor pump for presta,
All P/V stems have a smooth band on them, ( it is a common machine practice
to do that between 2 different thread types)
The gasket wear is near none , on my bell shaped air chuck,
since that self inflicted, forehead dope slap , had me change that practice.

Lezyne floor pump flip over heads depend in the fine thread in the stem being there

in the shop I see a lot of those smooth shaft stems fail at the bonding ,
because of wear and tear shoving the pump head on, w/o ring nut as back up.
[or, perhaps riding the tire under-inflated]