Have you reviewed the Pilot User Guide regarding the "Level" button? Here's what it says:
Level Mode
The Autopilot’s Level Mode (or Straight and Level Mode) will immediately attempt to reach zero vertical speed and a roll angle of zero. It will not attempt to fly the aircraft to any previous altitude or track and it will not respect any bug inputs.
Note that it does NOT go into "ALT" mode, where it holds an altitude - it just tries to zero the vertical speed (didn't we just have this exact same discussion here a couple of weeks ago?). And that zeroing the RATE of altitude change does not result in a constant altitude, as vertical air movement can result in altitude changes.
The LEVEL button is to get you out of dangerous situations. Once the plane is level, you will need to then tell the A/P what you actually want it to do (or disengage it).
R.... Nope - I won't say it...