It's Amazon. The Competitor is also hit with it, I haven't been able to download higher quality audio with them, they're also restricted to AAC 2.0. According to them, Amazon is making changes. Apparently the audio is there, they just can't grab it.
Weird thing is, I can still download EAC3 with 6.1.6.1 and 6.1.6.6 (for whatever little videos that can be downloaded). I tested those videos with the competitor and they only have AAC available while Streamfab downloads EAC3. So right now if a video on Amazon that can be downloaded with the aforementioned versions I'd download the audio with Streamfab, download the video with the competitor, then multiplex with MKVTool.