While I cannot argue that it makes no sense, I can confirm that for me it worked. To confirm this, I went back into my routes and enabled OpenCNAME again, and once again my DTMF failed. Remove OpenCNAME again and bibbidi bobbidi boo, DTMF works.
I cannot explain why, I can only share my experience in hopes that it helps someone else. One man’s fluke is another man’s treasure…