Be sure that the tank relief valve is just not open.
There is a (usually) slotted screw on the side of the main valve body that the attendant will open just a bit while filling the bottle.
Once topped off correctly the small screw is closed and the filling process is stopped. The tank is then correctly filled and an expansion area remains in the top of the bottle for expansion.
Your bottle may have an Allen screw for this purpose and it just did not get closed completely and the higher elevation with some warmth is causing it to bleed.
If you have the proper wrench, give it a bit of clockwise turn and thd problem is fixed.
Enjoy your outing.