It doesn't really matter whether the "Cube is correct" or not. The fact is that Amazon isn't going to change anything. They have known about this since at least last October and, despite several firmware updates, they still haven't done anything to address it.
The fact is, that its very likely something could be done on EITHER END to resolve the issue, and, if Amazon isn't going to address it, then Tablo should step up. Figure out how the other H.264 encoders are doing it successfully and then implement a similar profile.
You can go and blame it on Amazon all day long, but if they are not going to "fix" it then it will be up to Tablo or it simply won't get done.
There are lots of options to the profiles and features available for H.264 encoding. You can take a look at a list of them here (see Profiles/Features section): https://en.wikipedia.org/wiki/H.264/MPEG-4_AVC
Perhaps the reason other media streamers are not having the same issues as we see with the Tablo devices is that they have chosen a different profile or feature set for their H.264 encoding. Tablo could certainly do the same. Modify the settings in their profile for better compatibility.
There are also multiple suppliers for the codecs that these devices use. No one writes their own codec. They are simply bought or licensed from third parties who specialize in this capability. You can see a list of some of the more popular software implementations in the same article (under the software section). Maybe Tablo simply needs to compile/license a Codec from a different vendor. (If implemented in firmware/software).
...and curiously enough, Tablo just announced in this thread that they are testing some changes which appear to have mitigated the issue! ..changes which are "getting closer to a fix that should allow everyone to enjoy their Fire TV 4K". (IOW: A Software Update)
Who would have possibly thought that Tablo could do something to fix this issue on THEIR end?