Adding a class to the Execution Order list does not affect classes that inherit from it. You’ll need to add each individual subclass to the list in order to control their execution order.
Yeah I know, only a year too late. I was wondering the same thing but happened to be in a position to test it easily, so now people can know the answer going forward, at least.
You can put [DefaultExecutionOrder (100)] above your base class. This way without using Script Execution Order window, you can change the execution order and this way all the derived classes will have this order.