Tapestry validates all onXFromY() handlers to ensure that all Y's actually exist. In your case, it looks like Y does not exist in the child page since a different tml page is used. I can only assume this validation was introduced after 5.0.
I consider actionlink to be deprecated, you should use eventlink instead which is better for reuse as it allows different components to emit the same event. If you were to switch to eventlink and use different events for the two cases, the handler could exist on the parent page without failing validation.
