Argggghh! I hope the 1.2.1 leak is not the final release.
It doesn't seem to recognize using the 1.4x TC III with the 70-300L! (and it fits perfectly at the long end)
It doesn't seem to recognize the TC is on

.
It doesn't report it as a lens + TC combo for micro-adjustment and it says f/5.6 instead of f/8

.
The AF is awful! It does a hyper oscillation before settling in (only 1/3 of the time is it actually in focus

) and it's so slow.
It seems like they decided the 1.4x TC III is not compatible with the 70-300L so they don't let it work with the new f/8 AF mode

. Even though it fits perfectly fine if you keep it at the long end.
Even if they don't official claim support for the combo for fear of someone crying that they tried to go to 70mm and it hit it would be nice if they would just let it work even without being on the official list!
It was precisely with this lens that I cared about f/8!! I have my 300 2.8 that works fine with either 1.4x or 2x III TC already but that is so heavy so it would'be been nice to get f/8 out of the 70-300L for the times I don't wanna lug the beast around which can be a real pain.
Ironically, now I guess I need to spend another $250 and get a kenko? Maybe I should dump the canon 1.4x TC to pay for it.
But people say the Kenko is not as sharp with the 70-300L (or other lenses) and that they notice a bit of a quality difference with the 70-300L. What a shame.
I hope the final release version firmware is different or they allow for a patch.

It's quite a tremendous shame since the 70-300L actually maintains pretty solid quality with the Canon 1.4x TC III. At f/6.3 (f/9) the quality is relatively solid and it is definitely better than upscaling the bare lens, absolutely. It would be soooo useful! COme on Canon THIS is exactly the kinda scenario we wanted f/8 AF for! But seemingly being locked out of the new f/8 mode the 70-300L + 1.4x TC III AF is such an utter colossal wreck

. But the optical output is solid enough to be usable so it's frustrating.