I'd say that there is no such statistically significant data.

Practically nobody teaching K-12 has subject-matter masters degrees. It's just not part of the career trajectory. As unusual as a nurse having an M.A. in history or something. Yes, would occur on the margins of people changing course in life, but not the mainline.

Specifically, the question here is about the efficacy of pay-scale bumps for Masters degrees in education. To your point (and my counter-point), teachers get a substantial pay bump* if they hold a M.Ed, but no bump if they hold a masters in their teachable areas.

For persons who can afford it in the moment, taking a one year or two or three year part-time M.Ed. after getting a few years teaching experience (an entrance requirement in most M.Ed. programs) can pay for itself over the next 2-5 years, then is all surplus for the rest of the career.

* - all of the varies a bit by jurisdiction but I think this is "the general case".

My wife is a teacher. She wanted to teach history, so she had to get a history degree with a specialization in education. But there were no jobs available, so she accepted a conditional as a special education teacher. That's what drove her to get a master's degree in special ed.

While doing teaching special ed she developed a fondness for teaching math. But she isn't allowed to take on a general ed math class because she doesn't have a "math endorsement" - which would require her to go back to school again for basically another advanced degree in math. And she can't get a general ed job in history because it's too competitive and her years of experience makes her too expensive compared to fresh blood.