Here is the changelog, linked in post #2.
This probably happens all the time, but I don't usually pore over release notes. Just took a look and noticed this reference for the 86U:
Merged SDK + binary blobs from 384_81846 for RT-AC86U
Does that version number relate to the stock firmware. From what I can see a 384.81846 version was never released for the 86U and it went 384.81369 to 384.81858. So, I was just curious how this translates to the Merlin build.