I also tried this startup procedure, same result.
However, I also tried a different thing: I bound the “Set Avionics Master” control to a key and while starting up, after the generators are on, I applied the Avionics key and then the Multi Panel AP controls activated.
So, for me it’s clear that somewhere Asobo bound the activation of the Autopilot controls to the avionics, at least for the Cessna Longitude (didn’t test other planes on this yet). As there’s no avionics switch per se for the Longi, this never gets activated.
I will try to open a bug report, but as apparently I’m the only one who seems to have the issue, I see barely a chance that it will be fixed in the near future (other issues which are much more common didn’t get fixed yet, either).
Thanks for the infos and have safe flights