The reason for this behavior is that the name of the base FB "AeMesFu_xAddon" is identical to the default name of the derived FB created in your project. When the type definition in the project is created, it looks for its base type by name and finds itself. As circular base type references are impossible, the property for the base type remains empty. After renaming the instance to AeMesFu_1Addon, the names are not ambiguous anymore. When the object is reloaded now, the base type will be correctly set. (This can be triggered either by reloading all object definitions or by changing an object parameter of the addon, e.g. FuNo.)
That means it's not the export that is not working correctly, but the base type is really not set in the project data when instantiating the addon the first time.
A possible solution for this problem: Change the name of the addon to AeMesFu_x (whithout the Addon suffix). This is used as default instance name, so the name is not the same as the base class anymore and everything should work. Most other addons don't have that suffix in the name, anyway.
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