The problem only occurs if the symbol of the subcircuit has been customised in the main circuit.
Scenario:
Create circuit "Inner", input "A"
Create circuit "Outer", add subcircuit "Inner"
Customize symbol of "Inner" in "Outer" using "Edit symbol"
Change input name in "Inner" from "A" to "B"
Open "Outer"
=> Unexpected error
The subcircuit reference in "Outer" keeps references to the port in "Inner" using port names. For the custom drawing created when editing a subcircuit symbol with "Edit symbol", those references cannot be resolved anymore.
The problem only occurs if the symbol of the subcircuit has been customised in the main circuit.
Scenario:
The subcircuit reference in "Outer" keeps references to the port in "Inner" using port names. For the custom drawing created when editing a subcircuit symbol with "Edit symbol", those references cannot be resolved anymore.