Last night, having already made 2 successful flights, I decided to casually fly from one airport to another a couple of hundred miles away after closing down the default state, and doing an APU start-up.

Assumed fuel was what the default state had onboard.

But climbing to FL350, the plane in AP,on Heading Track,on manual V/S, all of a sudden,decided to descend at 21000ft.

Any reason why it should do this? Just for future reference as all previous as per CDU prepped flights have so far been perfect except for a black overhead screen when trying night flights