Hey friend so as you have even noticed your self the problem is localized to where your are exausting your air into the vegetative tent to utilize all available resources . That stale air coming in and the heat will cause what looks like like heat issues or even a deficiency like cal mag which you haven't added yet . If you can have the air moved in just not direct on top of plants may help ... Do your cal mag and move that air exchange a bit and it should help ,I don't believe it's rust fungus but lower humidity would help eliminate that possibility in the future ...cheers and good luck brother ..