I know this was reported a while back on Steam. I submitted a bug report.
I started a new game on the latest Alpha today. There is a random bug affected medium stand aircraft when baggage handlers removed baggage will not remove any baggage and just start to Spaz out. While passengers load OK, it then also fails to load baggage with the loaded baggage truck queuing on the stand, waiting for the empty unloading baggage truck to GTFO, and while the pushback truck appears it also fails on the pushback, so the only option is to dismiss the aircraft without baggage being unloaded or loaded. Saving and reloading seems to fix for a short while, but then it starts happening again.
Thanks, it’s a known bug that we’re working on. A fix for it will be out by next week on the experimental branch, there’s a few reports on it now and I believe we have enough data to get it squashed. Right now the only work around is either a save and reload or simply not to use belt loader trucks.