key factor to take into account is that this is reported trafficking. If legalized sex work means more light is shed on human trafficking that means more can be done about it.
Just because more is reported doesn’t mean more isn’t also happening. In fact, one could reasonably expect reporting to go down as a percentage of incidents due to ordinary citizens not expecting sex workers to be involved in trafficking since sex work is now legal. That the number goes up after the stigma is removed seems to strongly indicate a correlation with a rise in actual trafficking.