Trouble Animating Custom Model

I've managed to get through several YouTube tutorials on the Animation Rigging package and animate some free character models. Now I've made my own rigged model and am trying to get the same results with it.

Everything works fine until I try to add a Two Bone IK Constraint in Unity. When I press play, the character appears high above where it's supposed to be, and moving the IK target produces further strange results.

Since I've been able to get Animation Rigging to work with other models, I'm sure there's something wrong with the model I made, or something in my scene setup, or even both. But after multiple attempts to fix the problem, I'm not sure what to do next except post here with a link to the project files.

If anyone is generous enough to take some time to look at the project and/or model, it would be greatly appreciated. Observe how running the scene causes the unwanted behavior, but simply deleting the RightArmIK object makes the animation play correctly.
https://drive.google.com/file/d/18HPOR99OJrj19fKXy1d7BAyA6vnTB4J_/view?usp=sharing

(Please note that the project uses the Animation Rigging package. Let me know if this isn't the correct way to share a project, and I'll gladly update the Drive link.)

 

A respectful bump. This issue is starting to hold my project back, so I'm really hoping for a reply soon.