Just an observation it seems planes using aigtc will spawn at your current altitude when you’re on the ground they get bumped up 1500ft or so above the ground level.
I started a flight forcing my plane to spawn at 25k ft and every enroute plane spawned at 25k ft as per the aigyc window they then set about ascending and descending to their given altitudes.
Also when you get far enough away from airports planes never gain altitude after takeoff and just fly along a few feet off the ground (or so it’s being reported by aigtc).