For reference:

Season 1: Amazon Prime Video (also, Netflix)

Season 2: Amazon Prime Video Channels

3-5: Prime Video

6-13: Prime Video Channels (with 10-13 also available on the Roku Channel)

14-19: Prime Video (with 17-19 also on Netflix)

20-22: Prime Video Channels (and Hulu, and the Roku Channel)

23-25: Prime Video (and Netflix)

So, they're all on Amazon in some sense. I was aware that there was some kind of concept of Prime Video Channels, but when I tried to find an explanation on Amazon's website, I failed.

Following up, "Prime Video Channels" seem to be an Amazon offering in which you have your subscription to a separate video service (the "Channel") billed through Amazon, for "convenience". (And you can also watch their stuff on Amazon's website.) So Pokemon has licensed about half of their series to Amazon, and they reserve the remaining half for people who subscribe to the Pokemon Channel.