I think I MAY figured out the issue. For those with Simbrief, if the existing plan does not match the starting location or aircraft, the sim never loads due to the below error.
Umberto/Joe - can you guys test and verify?
Icao airline was None
Icao airline reset by Simbrief to DAL, language ('', 'mixed1')
True KORD KMSP KORD
SimBrief aircraft A320 doesn't match MSFS aircraft A20N
Exception staticGateObject destroyed or None <Couatl.SimObject object at 0x00000239C10203B8