[EC900]

I agree, it’s not the gas cap. You could have developed a vac leak, more probable.
Using any fuel cleaner additive? Sometimes this can help if the injectors are a bit dirty.
CIS is in my opinion more finicky, curious being a CA car has any work been done to meet a CA BAR emissions test?

For what it’s worth, the gas cap has an internal mechanism that prevents it from being over-tightened and will click when fully seated. The tank itself is vented thru the expansion tank, you will see the hose on the top of the tank next to the filler neck inlet.