My cap had a pressure/vacuum relief, but I defeated it while attempting to solve that vaporlock problem. I doubt your cap has changed or suddenly started to malfunction. The hissing was the cap doing its job.
The bubbles coming out of the bottom of the tank indicate gas vapor being generated, which is pressurizing the tank of course.
Both of those points you already figgered out. More importantly, it's not killing your engine and forcing you to take beverage breaks (yet).
And the big question you've already voiced: why now!??!?
My guess is a lower quality of gasoline in your area. Here in northern CA it didn't matter what brand or octane I tried, it all acted the same. But refiners are switching to "summer blends" now, and this summer's blend for your area could be more volatile than before (is that the right term? "boiling point" didn't seem like the best term for gasoline).
Even though I've moved my fuel system to the right side away from the exhaust, I'm still getting some VL, it's just not killing the engine nor bubbling up into the tank. I can hear the electric pump and the sound is different whether it's pumping liquid or fumes. It happens now after I've been working awhile and then shut down for 5 minutes then go to restart.
On hindsight I'd follow the suggestion of buying and installing some of the high-dollar fuel line insulation made for racing. (Can't remember who tried that successfully at the moment and it's too close to "go-home-o'clock" to look back -- BobRip or TracDoc if I was to guess but that's probably incorrect).
I have learned that there is a chain of events that occurs: once the fuel starts to VL, then the engine starts to run more lean. Running lean causes it to run hotter. Running hotter causes more VL. More VL causes it to run hotter.....
Although I haven't had mine die from VL since the last mods, I'm not done. Actually, I think some of that fuel line insulation could eliminate the last of my problem. I can't remember how many feet come on a roll, but if you find a deal and wanna split it, I'm game.
Phil