Hello,
I first noticed this happening on the "Millenium Mines" scenario where suddenly a load of puke appeared by the ride exit.
So I created a test file in Forest Frontiers (which ive attached). I created a miniature railway with 15 circuits and made it free, used the "Large Tram" cheat and placed a No-Entry sign by the entrance.
As you can see by the image, a huge amount of guests get extremely sick seemingly out of nowhere, and I cant work out why it's happening. I have tracked guests entering the ride with 0 nausea and then leaving the ride very sick, so it's definitely the miniature railway. What I think contributes to it is increasing the circuits mid ride, but I'm not 100% sure.
EDIT: For clarity, the ride has a very low nausea rating of almost 0
Forest Frontiers SICK.park