I had just such an insidious leak once. I sprayed every threaded
connection to no avail. Know where the leak turned out to be.... the
seat of the pop-off valve. I had not thought to spray the entire
pop-off valve, I only squirted the threaded connection and totally
missed the problem area on my first few searches. Sure enough, bubbles
formed at the side holes of the relief valve.

I guess the moral of this story is always look beyond the fittings.
Even things like pressure switches can leak internally (like if the
diaphragm is old). Your leak is A LOT faster than the one I had, but I
still had very obvious bubbles once I found the spot. My leak also
stopped at around 80 psi, with the pop-off valve spring winning out
over the air pressure. Only at high tank pressure did I have a
detectable leak.