That would help verify my hypothesis but since your problem is already gone there's no way this could help you :-)
So if you can check this or dig the logs from somewhere I'd be very obliged. Speaking of PCIe device probing, nothing has changed in upstream kernel or OpenWRT patches yet it started working for you. I have two supposedly identical R6220 devices: one worked flawlessly and the other (the same kernel version) would not initialize pcie2 card in ~90% cases.
I strongly suspect there's missing delay in new driver but devices are probed in parallel so actual intervals depend on what features are enabled and what devices are actually connected.