"Passing through" fast USB devices into virtual machines does not work, as USB has no real (virtio) pass-through option (contrary to PCIe on hosts with an IOMMU). While this semi-emulation kind of works with slow-paced devices (e.g. NFC readers), it utterly fails with any USB devices (e.g. wireless cards) that depend on fast data transfers and timely (semi-realtime) responses. In some cases it may be possible to pass-through the whole USB host controller (on the PCIe) level through into the VM, while this approach would work, it may come with rather huge collateral damage (as all USB ports of this controller will then be passed through into the VM and are no longer available to the host).
tl;dr: you can't pass through USB WLAN cards into virtual machines, trying to do so will fail - spectacularly (crashes) or more subtly (weird failures, instability, etc.).
This is nothing that could be fixed at the OpenWrt side, it's a technical limitations of how USB works (and to a lesser extent the hypervisor).