Variable starts with underscore (_) cannot fill value correctly

Tried to prepare environment variable like (e.g. _TEST), and Yaak seems to recognize it as below screenshot:

However when actual request is called, only the first variable test1 can be filled automatically, while test2 will remain variable like:

--url-query 'test1=test1234'
--url-query 'test2=${[ _TEST ]}'

Thanks for your nice work!