Not had this problem, but my TT tank (I got one in the end) is plumbed into the breather of the main tank and I haven't been to 8000 feet yet. I'm assuming this is happening when you fill up at low altitude then climb up high?. The only thing I can think of that may be causing this is some kind of obstruction in the vent on the main tank. As you climb up and the air pressure drops, the expansion of the air in the main tank may force fuel back into the TT tank. If it happens when you are starting at altitude it could be temperature changes causing fuel evaporation in the main tank, which would occur at a lower temperature because of the altitude.
Check the vent line from the main tank for obstructions. There is also a check valve in the top of the main tank which prevents fuel entering the breather line if the bike tips over which could have got stuck. No problem to remove it. Undo the 6 screws securing the white plastic breather hose spigot in the top of the tank. Lift it off and discard the spring and white plastic float. Did you follow the TT install guide specifying the breather had to be rerouted to the front of the bike above the height of the TT tank? With the float in place this is probably not necessary but with it removed it will be important.
Good luck, enjoy your travels.