Holding A by default sets your character's velocity to max in the up direction.

If you also hold a direction on the left thumbstick with a non-maximum amount, your movement direction is as expected (Up + Stick), but your speed is the speed you would be walking at with a non-maximum thumbstick position. e.g., hold A + (Stick 10% Right) and you will move Up+Right at 10% speed.

This is non-intuitive to me, as nudging the stuck while flying straight up suddenly decreases my speed, sometimes to near 0, when what I want to do is move in the direction of Up+Stick at max speed.

Put another way, as the user I expect that directional inputs I am giving my character will be summed, at least up to max speed, and wouldn't ever expect two direction inputs in non-opposite directions to slow my character down.

Hope that made sense :)