Er, awful was a harsh way to put it... :/
But, it is kind of pathological. I will further destroy my thumbs on my phone keyboard for the sake of insomnia. The tarped in plant area has the humidifier and is at saturation point inside from time to time. Condensation waters the plants but also beads on the plastic, settling on cracks, seams and folds in the plastic because of surface tension. The liquid water has a different ability to conduct heat than plastic, particle board or air so it becomes relatively cold. Because it is cooler than the air, and the air has an excess of humidity, water vapor is absorbed by droplets as the vapor cools, creating a sort of precipitation effect. Because those droplets settle into the nooks and crannies an cracks in the tarp, they act as a sort of freeway for water vapor that wants to leave the enclosure.
Now, there ar wrinkles on the inside of the tarp, and as a result also the outside. That means the same type of folds drawing water out of the enclosure on the bottom have a spot that's shaded and relatively free from air currents where water is pooling. (It likes to pool up in the wrinkles because of capillary action.) Add that to particle board, an excellent source of energy-rich cellulose, and it's a good place for molds.
In addition, you have a plant light that gets hot. The hot air rises above your lamps, and stops at the ceiling. Then it rolls from one direction or the other to get out of the way of the air that's still being pushed up by the lamps. Unless you have a very cluttered or turbulent room, it's likely that there's a sort of circular draft of air that blows up the wall your lights are against, up to the ceiling, across to the opposite wall, down and back along the floor to the space underneath the lights. Or something like that if there are other heat sources in the room. In any case, the air around the vase of your enclosure is likely to be a continuous, very gentle flow of the coolest, wettest air in the room. Because the base of your container will be cooler than the top, even with perfect seals this is another opportunity for condensation.
Or I might be wrong.
~Joe