I've been now playing for a while, tried different situations and kept pressing shift at the end of every traffic stop. It obviously never really canceled anything since the driver always drove away after issuing a warning or a ticket. I've also tried cancelling it mid traffic stop, which worked always fine. It got cancelled and I could stop somebody else right away. I've tried using speed checker again and all worked fine. Then I pulled behind a driver who was on his phone (created by traffic policer), stopped him, got him a ticket for using a mobile device (through patrol toolkit updated with traffic offenses), then issued a warning through lspdfr+ traffic stop menu so that he could go away (you have to do that if you're using patrol toolkit since the two are not really connected together), he drove off, I got back in my vehicle, pressed shift again (just to make sure) and after this stop I could no longer stop anybody else.