Without roof decking your attic was most likely colder / less air tight than it is now, therefore, there was no condensation and mold. Assuming that you have proper type of vapor barrier for you climatic zone, sufficient amount of insulation, enough and correctly distributed lower and upper vents, and properly balanced air flow between the soffits and the ridge vent, the next step would be to increase that air flow.
Some attics, especially in colder climates require more than the minimum amount of ventilation. If there’s no room for additional vents (you already have continuous soffit vent and full ridge vent) you may need to go with a humidistat and thermostat equipped attic power vent
Unless your access panel is heavily leaking conditioned air into the attic space I doubt it would be the only reason responsible for the mold growth.