Cessna 208 B Gran Caravan stall and speed

I experienced the same. I never flew a caravan in real life, so I can’t compare. There are some threads about AP problems with caravan, so they might be related. For me it helped to manually stabilize the aircraft before engaging autopilot, but sometimes it just threw the nose up anyway. Retrimming and turning autopilot off and on again also helped sometimes. There is definitely an issue with it.