By design. The text-only emails are not created by evaluating the Comcode to HTML then converting it to text, they're created by stripping down the Comcode as much as possible.
Comcode is primarily a markup language, so it's outside the design considerations for Custom Comcode tags to be used to insert content, at least outside Comcode pages.

I'll add a new issue suggesting to make a text-version Custom Comcode tag body field.