I don't think the core has any template events inside of block_var_array outputs, but I was working on an extension, and had a temporary template event of my own for testing purposes that did, and it has been broken by TWIG.

So if you have a template event set up in a core template such as:

<!-- BEGIN topicrow -->

	<li>

		<dl>....lots of stuff from the core template file...</dl>

		<!-- EVENT foo_bar -->

	</li>

<!-- END topicrow -->

And your template event file "foo_bar.html" has the following:

<!-- IF topicrow.TOPIC_TITLE -->

	<strong>{topicrow.TOPIC_TITLE}</strong>

<!-- ENDIF -->

Nothing gets injected into your template. TWIG seems to get confused by the looping "topicrow" variable. It does not correctly acquire topicrow.TOPIC_TITLE and so injects null.