Like you, I'm on AC68U hardware (I have the AC66U_B1) and also having problems.
Unsure if related but I never ran the beta but I'm getting non-stop RDP disconnects this morning. The same thing happened on 386.4 until I did a factory reset. With 386.5 I started off the right way. Upgrade, factory reset, enter configuration by hand. All seemed fine on a very basic level, but this morning until I started my daily RDP session to my work machine is disconnecting constantly. Not much I can do.
My desktop that I RDP from is on ethernet, but my RDP "server" is a laptop on wifi. On top of that semi-demanding task, no two wifi environments are the same. I'm very likely in one the most challenging wifi environment of anyone here, living in a high-rise building. Every band is saturated a few times over, and I think that's why I'm seeing performance gaps between various releases that others aren't.
Not only is my RDP reconnecting constantly, my work VPN is doing the same- but only when I'm connected to the machine through RDP.
That said, while MSTSC is unstable, Teamviewer is not. I believe I have Teamviewer configured to route over the LAN so it's not going out and back in. But that's only sending 1 monitor worth, while RDP is configured to send 3 screens worth. Big difference in traffic.
So this isn't helpful but I thought I'd get it out there. I'll try another router + computer reboot, but if it doesn't fix it, I'll probably go back to 386.3_2 once my wife is done with her work at noon today. She teaches kids all day so there's another daily test put on my AC66U_B1. Never had a single issue until I left the 386.3_x branch though.
edit to add- It seems to have settled down now.. will post an update if I have to end up reverting. I statically assign both of my machines involved with this RDP kerfuffle. Of note, I do see a lot of these in the logs to my laptop if it matters-
dnsmasq-dhcp[1275]: DHCPREQUEST(br0) 192.168.1.4 xx:xx:xx:xx:xx:xx
dnsmasq-dhcp[1275]: DHCPACK(br0) 192.168.1.4 xx:xx:xx:xx:xx:xx
edit 2- One of our video livestreams are freezing (on wifi). While my disconnects cleared up... I noticed my audio cutting out in a meeting too. I can't have many issues (hence the attempt at some insurance with a factory reset), going back to 386.3_2 where it seemed like the wifi drivers were best.