The humidity may or may not be a problem. Younger vegetative growing plants actually like fairly high humidity. The only time you don’t want the humidity too high is during flowering as to avoid bud rot, but you aren’t even getting that far.
What do you mean you start them in soil and then put them in a net pot? If a lot of soil is still on the roots when you put them in a net pot, in a very wet form of hydro, this will usually cause the roots to rot and or you could get the trunks of the plants rotting as well as the soil will keep the environment way too wet. Cannabis roots need lots of oxygen and can easily get waterlogged roots not unlike cactus roots.
What type of hydro with net pots? If it is a Deep Water Culture bubbler, you certainly need to carefully remove as much soil from the root ball as possible when preparing it for the net pot and the DWC otherwise you will get these problems, the plant may grow fast for a bit but then as the roots and such start to drown and rot away you will get a form of damping off and everything will wilt and die. If you are using some sort of flood and drain then you may need to let the soil and media in the net pot have more time to kind of dry out before it is re-watered so as to get that air/oxygen to the roots.