I did test with a usb3 thumbdrive and it didn't mount either. But wifi was enabled.
I just found out yesterday that NFS, specifically nfsd isn't present in 18.06.2 and is apparently a known issue. I was going to use that to backup my linux clients, rsync again. Bummer.
I bought a spare, used R8000 off eBay, which I planned on using to test upgrades. In this way I'd always have a working router with the previous release. Both are currently running 18.06.2 now. I can use the spare to test the USB3 with no wifi. Although if it works I'm still stuck, since I need wifi.
I soft bricked one of the routers and had to use a USB-TTL cable to load via tftp. That experience gave me a warm and fuzzy that I'll be okay experimenting.
It's the 2.4 GHz band that's know to interfere with USB 3.0, as their frequency is close. You could just start by disabling the 2.4 GHz radio and keep the 5 GHz for access if cable access isn't convenient.
I have the exact same problem. I can mount an USB3.0 USB drive (32Gb) but I cannot detect my Seagate 6TB using USB3.0, the only way to detect it and mount it is by using the USB2.0 port
I'm using OpenWRT 18.06.2
As the OP, I never did solve this problem. I intended to try a snapshot to see if it fixed the issue but never got around to it since this drive only receives backups from two computers while I sleep the speed wasn't a priority.
Version 18.06.03 and probably 19.07.00 will be out very soon, I recommend waiting for them to be released and flashing one to see if the problem has been corrected. I wouldn't waste a lot of time trying to figure it out on 18.06.02. When I do flash one of the newer releases I will retest this issue and update this thread.