The only possible explanation for this to be happening is:
- Captain sim is interfacing with GSX (we don't know if they did it, but the procedure is fully documented in the manual, so they don't require our support to do it), and is linking its electrical system to the availability of a GSX jetway with a power unit.
- The jetway disconnected, so you lost power, because you should have turned on the APU before disconnecting the jetway.
The latest update, in fact, IMPROVES on the previous version, and delays the automatic jetway disconnection until you call for departure. With the previous version, it disconnected as soon boarding ended, and this caused a problem with airplanes (like the Leonardo Maddog X), which we know are interfacing with GSX to get electrical power from jetways. But I have no idea if Captain Sim does the same for the 757.