Couple of points:
Marking infractions are a specific type of call that are designed to allow the marker to make a call and for the stall count to be reset without a stoppage of play. Once play has stopped, that no longer applies.
A “foul” is a breach that involves “non-minor contact” – so that could not apply to a fast count.
But a violation is applicable for any other breach.
So once play has stopped, and there is an accepted fast count, it is not applicable to reduce the stall count by 1, instead 9.5.1 applies as a an accepted breach by the defence has occurred.
The same would apply for any other marking violation called under 18.1.5.
Hope that helps.