IMHO, you should have kept everything in the same thread instead of making a new one every single update w/o proper explanation in text.
and 3 URL isn't going to make them understand what you want, especially with new thread.
so if I follow your latest video correctly
up to version 2406: you had no issue with the memory running properly w/ both Ryzen 2600x and Ryzen 1200; which is before you receive the 3600.
when you received Ryzen 3600, you installed the CPU w/BIOS v2406, and the issue started, had nothing to do with the BIOS version but the fact you installed a new CPU and it happened? It just still happens w/ v2704 n v0002.
So have you put back the 1200 with the v2406 or v2704 and test again after dual-channel fails w/ 3600? that's not clear to me.
From there, I see 2 possibilities, assuming you did put back 1200 and it gives you back the access to both channels.
1: Asus did f**k up the BIOS, 2: somehow your 3600 has bad memory controller? maybe bent pins or bad pins (something like that to cause that particular channel to **** up)
If putting 1200 does back gives you access to dual-channel, you can just sell the 1200+ Asus board and grab a new board from a different brand.
However, I will be more worry about the 3600 being bad and will have the same problem on different board.
maybe try PM Raja@Asus or SHAMINO directly to get their attention too.