That number rang a bell. I’m sure I’d read that before. If you search the forums for that you will find exactly two posts for “99900”, and both of them related to the autopilot.
From that I suspect it is not the plane, but something in the AP logic that you are triggering. There are some other references to “99000”, but again all to do with the AP, and altitude capture.
Another post suggests this could be because a couple of values might not be set in the planes “systems.cfg”, which are encrypted and therefore unavailable to check…unless those values can read by SimConnect.