Construct picks an arbitrary existing instance to copy the properties from.
If you have two instances with different properties, and create a new instance at runtime, which should it pick to copy properties from? There's currently no way to tell Construct which one to use as a reference, so it guesses one. It could use either.
The main idea behind this is that if all your instances use the same properties, new instances get the same properties like you'd expect. If your instances use different properties, you can use actions after creating the instance to set the properties that matter.