The dropping speed - could be  rip in diaphragm. If its gets any larger than a tiny pinhole the bike essentially wont even get to 20. Its gotta be perfectly sealed.
The Vent wont splash fuel, till you drop the bike. Then its gonna clumsily dump it on top of the carbs and motor, instead of clumsily dumping it by the airbox, which actually is better, cos those parts aren't hot ... till it hits the exhaust.
The 01+ airbox has a blocked off vent and a vent with a hose in it. Sorry, I was thinking 89-00 airbox.
Solenoid that weird crap on the throttle butterflies ? yea study it all you like, it doesn't do crap.

Cool.
Buddha.

A rip in the diaphragm sounds exactly what would match my symptoms. Especially considering the worsening progression. I'll try to do the take apart for inspection over the next few days and report back.

Did you guyz find the culprit for this issue?

I think my bike is doing it too and started overnight.