I use it with AWZ3 just fine, but then I also haven't allowed my Kindle to update its software in years, so maybe Amazon is giving me files with older DRM.
I bought a few books a couple of months ago from Amazon for use with their PC viewer and the first thing i did was to remove the DRM (so i can read it on my phone using my preferred ebook reader... and i trust myself more than Amazon to keep the books around) so i think they managed to break the latest version too.
Now that is interesting, I have a kindle oasis and mix between reading on that (which is glorious I must be honest) and on my iPhone.
If I can download a “legacy” file format - I might try that.