Very odd?

I would have said there was a leak internally in the breather pipework, but as you've had a new tank, that seems unlikely? If it only does it in hot sunshine them it can only be fuel expansion, or fuel vapour/pressure build-up from evaporation. Even this doesn't sound possible if it does it with a part-full tank?

Try this... Leave the bike in the sun with the filler cap loose - not locked. See what happens and report back.

__________________