This is by design because the Add Extension option does not do the same thing as the Procedural Components options.
The "Procedural Components" dropdown functions use a "set": when you select a thing, it replaces whatever was there before. Those dropdowns are shortcuts for what you can equally do using the "Add Component" button - just less mouse clicks.
The Add Extensions use an "add": it adds a new component without replacing what was there before. You'll notice that in the Add Extensions, when you add one and then bring up the menu again, the thing you added is no longer selectable.