FWIW, I've been using arbitrary master revisions on a wrt1900v1 for quite a while without noticing any such issues. I have a single FS USB device plugged into the USB3 port, and do use this device regularily so I am pretty sure I would have noticed it failing. I believe FS and HS devices should behave the same way wrt USB3, so I don't think that difference can explain this working for me.
BusyBox v1.30.0 () built-in shell (ash)
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OpenWrt SNAPSHOT, r9330-880f8e6d3287
root@wrt1900ac-1:~# lsusb -t
/: Bus 03.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/2p, 5000M
/: Bus 02.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/2p, 480M
|__ Port 2: Dev 2, If 0, Class=Vendor Specific Class, Driver=io_edgeport, 12M
/: Bus 01.Port 1: Dev 1, Class=root_hub, Driver=orion-ehci/1p, 480M
EDIT: A thought just occured to me: I have never tried hotplugging anything into this USB port. Maybe that is what's broken? My USB device is always plugged in. Could you please provide some more details on how you do the testing?
Right after I opened this FS I flashed a bot generated snapshot and the USB3 port had power with that image. I then reconciled the installed modules of that against my image and nothing stood out as a possible cause.
I am just leaving an old USB thumb drive (1.1??) that has an on-board LED to see power/activity plugged into the USB3 port.
Some [[https://termbin.com/td07|data]] from an image flashed today.