Sounds like the two stoke oil may be leaking into the gearbox oil. Have you noticed it using much 2T oil?

To check it, put a catch tray under the bike and open the gearbox oil cap. If oil floods out you have the well known pump seal problem.

If it's not that, then you may have a blocked breather causing an overpressure, but it's hard to believe since there's so little pressure in a normal gearbox. Could be an amazing failure whereby the crank is pressurising the gearbox!
_________________
MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball