help with humidity raising when lights on

i have never had this problem before but for some reason my humidity is raising when lights on :

i am running in clay pebbles and drip feeding for 5 mins 5 times a day got 1800w with 6" outtake running constant and 6 inch intake when lights on my tank is on the floor and moving it out of the room is not a option

RH is around the 70% mark with lights on , i have a dehumidifier running constant , i am extracting constant and bringing air in when lights only to also try and keep the heat down as now the temps are sitting at 84f . if i turn on another dehumidifier the temps raise to 90f .

after lights off i am getting temps of around 70f and rh at 52%

i am at around week 5 of flower the plants look very good but worried told is going to set. what i have noticed is that the plants seem to be really liking the high RH i have run similar set up with the same cuttings before but always managed to keep the Rh at around 48-52% in flower and thay have never looked so advanced as these do, i have done a little research and there are some that agree there grows also look better with a higher RH in flower but no one ever tries this.

any ides would help i don't want to risk losing this crop

Hmm, unless your dehumidifier is not doing its job once the room gets to a certain temperature? Is it an old dehu? Maybe a good cleaning, seems bizzAre forsure because those HID lights aren't going to make anything but a ton of dry radiant heat.... Atleast from my knowledge and experience.

Another thought is possibllllyyyyy the tables staying to moist so when the lights are kicking on it is rapidity evaporating the remaining water in the table and causing a quick RH spike.... Are you possibly flooding more often then you normally did? Get rid of an exhaust?

As far as keeping the RH high I have no experience all the way through late flower, WATCH OUT FOR MOLD that's all I can give on continuing with the high RH

i am going to try less feeds but don't like to cut it down in hydro as it then seems pointless , however needs must the only thing is maybe the vent out to extract is not as good as it should be .

If it hasn't changed and you've sucuessfully made it to this point before in the same conditions with a much lower RH then I wouldn't say that's the problem, unless it's went out.