my bdac2 is also a photo cell and only activates the regulator when lights are on. co2 will diminish after the regulator is turned off after a period of time, which leaves me to believe you have a leak at the tank/regulator. I may be totally wrong, but that's my guess. I don't go over 1500ppm when lights are on, as that's the only time plants utilize co2 absorption.
I use a dual hose a/c inside the room. it does suck out some co2, when running as proof from my walls and flooring being sucked in a tad. which I don't mind at all, as a refill is only 20.00 in my area and I have a back up tank, so I always have a full tank. this is why my unit reads a negative ppm reading after xx amount of hours at lights out. I cant compare a room with another a/c type (mini-split) that doesn't have a neg pressure when running, as ive yet to read about this scenario until now.
theres a couple co2 guys that ive been pm'ing for tips. one has helped me in my co2 thread, but I cant remember his name.