Unique prefab instances revert to original prefab values on play

Hey everyone!

I'm having a weird issue where I'm setting the value of an instance of a prefab in the scene view, then on play the value reverts back to its prefab value. It's also worth saying that I'm using a custom editor to work with vars, but our system previously worked in 5.5 so I'm not sure what is making the values reset now. The value, on play, seems to reset to the prefab value but not actually reflect that it's using a different var in the inspector - the inspector will show the "proper" value.

Any guidance here would be much appreciated!