Embed tag does not seem to work

I get an error when importing a template that references another template using {% embed 'component/button_group.twig' %}:

Template "component/button_group.twig" is not defined

However, the markup is correct because Twig (PHP) compiles OK and similar paths work with {{ include() }} in Twing.

Possibly related to #522 (closed) and #474 (closed).