Fonts won't embed on Kindle Fire

I have been writing to Kindle (and asking on Forums) for at least 5 years about the problem with embedding fonts in mobi files for the Kindle Fire and later. I have only one proprietary font that needs to be embedded. It is not, and instead, nonsense shows up.
For about a 6-month period a little over a year ago, files uploaded on the Kindle web page actually accepted the font. I was surprised and immediately prepared the book for publication. Then it no longer did and now I cannot update it or add additional volumes requiring the same proprietary font.
I have tried The latest Kindle Previewers as they come out as well as Caliber, but nothing seems to work. The fonts embed fine in epub format and the file I made in 2019 that was accepted displays perfectly on my Kindle Fire. So I know it is not impossible.
Also, no matter whom I talk to at KDP by chat or phone, no one has any answer, and most of them try to assure me that there is no problem. I even sent screen shots of the same page when it did work and when it did not. No answer.
Alas, there is no way to get into a mobi file as there is with epub files, to see what has gone wrong.
I don't see how difficult this can be to fix. At least it would be nice to receive some formal recognition of the problem...