Moreover, "Mixed Reality" strikes me as marketing-speak. Compare the search trends for AR [1] vs MR [2]. While both terms have been in use for a long time, AR was always more prevalent, with a notable rise in interest around 2009. My first encounter with the term MR was in marketing materials for Microsoft's HoloLens circa 2015. Now compare the search trends for MR [2] with HoloLens [3]. Searches for MR don't begin to trend upward until after the introduction of HoloLens, where Microsoft's marketing materials tried to make a big deal of said distinction and the rest of the world started to run with it. As a supporting argument for the original broader usage of the term AR, I'll cite discussions of a 2007 anime series, Dennō Coil [4], which explored various examples of fantastical AR usage. Probably just a coincidence, but the search trends for Dennō Coil [4] do precede the search trends for AR [1] rather closely.
Naturally, all the usual caveats about word meanings changing over time apply, but this one is a pet peeve of mine.
[1] https://trends.google.com/trends/explore?date=all&geo=US&q=%...
[2] https://trends.google.com/trends/explore?date=all&geo=US&q=%...
[3] https://trends.google.com/trends/explore?date=all&geo=US&q=H...
[4] https://trends.google.com/trends/explore?date=all&geo=US&q=%...