I'm pretty sure this is just an Ethan character issue, but it did take me some time for me to realize that my flat-cube areas were too tall, which caused the AI character to have these jumping/floating fits. After I made sure the baked Nav Mesh was flat, he would run on the ground just fine as expected. You mentioned some animation issues (like Ethan sliding down the ramp) before, so I'm guessing this is the same?
The Ethan character can be a bit weird to use at times as it's based off a few unique values. Also that character utilizes a capsule collider for detection. I'd have to take a look at it again, but I believe he might be using a Character Controller component which has settings for step height so he can climb up higher steps.