Hello again, using the recent release version I found an issue concerning empty array.
The cause of out of range exception is within the _GetPropertyList method.
Also happens with other modifiers that has joints array.
Viewing post in Joints array becomes empty
I looked into the issue, and I think it has something either to do with Godot C# loosing data when building OR because the data is not marshalable (serializable) it gets lost on rebuilds. There was a few issues about it on GitHub, but nothing definitive. I will do more looking into it soon to see if I can find a viable solution, but short term you may want to close and then reopen the scene file after building, as it looks like it may be a Godot bug.