The quickstart does work with RC5.1, so this is a regression.
A git bisect run shows that r1151831 for WICKET-3931 introduced this problem.
The problem lies in org.apache.wicket.markup.MarkupFragment.find(String). It stops looking for inner IMarkupElements if the component doesn't match and is not auto-component. I'll see how to make TransparentWebMarkupContainer's tag always auto, and what would that break.
Update Emond's patch by extracting the common code in AbstractMarkupSourcingStrategy.
Now it seems only FragmentMarkupSourcingStrategy (FMSS) doesn't reuse that common code for free.
@Juergen: do you think that FMSS needs it too ?
I'll modify the quickstart to a unit test.
I've heart Martijn is working on the test case too.
Assigned to RC 7, but fix should go into 1.5-RC6