I had seen several Miniplanes, including mine, with the same balloning effect.
Adding a 1/6" shim to the back wing mounting cured all. Also the right trust is mandatory to avoid groundlooping.
After this mods the plane will fly like a dream.
If anyone wants to buid this or any other biplane better, should buy the Harry Higleys book about biplanes.
He does a lot of good mods to the Miniplane that includes changing the top wing incidence by shimming the cabane mounting plate.
Is the book to have for biplanes.
Saludos.