[v2.7.6] [TeX input] \text inside \operatorname (sometimes) left in text node

This is a weirder one.

$$\operatorname {\text}}$$ leaves \text{ in the mtext node.

But, e.g., $$\operatorname {\text{a}}$$ is fine.

Reproducible with https://cdn.rawgit.com/mathjax/MathJax/2.7.6/test/sample-dynamic-2.html.