In flight worlds it is very common to open your Quick Menu while flying due to the typically large distances between players. Previously, you were able to keep control of your plane while having your menu open because Saccflight constantly pulled the Input data for controlling the plane.
However, in the beta build 1452 the Udon Input APIs were changed to not report any active inputs while VRChat menus are open.
Because of the change, the player now immediately loses control of the plane when the Quick Menu is opened. This results in an overall worse experience for new and experienced players is worlds like Test Pilots.
I am unsure what the reason behind this change is, but the easiest fix for this problem would be to simply revert the change. As an alternative, an option/toggle could be provided to world creators or users to choose what behavior is preferred.