Yes, it's fixed now, and it was a combination of two issues:
- We haven't configured the Q400 with an embedded stair, so it used a small stair.
- That small stair was still missing some important parameters which are required to use passengers.
You can get the updated files by running the Live Update and (this might be important), RESET the Q400 configuration, so GSX will use its (now updated) internal one, which has an embedded stair configured.