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