@joykiller check that your connectors in motherboard pcie sockets do not touch each other.
You mean the x1 connectors? Those are not touching, the USB wires are, but its been like this now for over a year and had no issues.
Yes, make sure the usb female connector of each x1 adapter is well spaced from the back contacts of the adjacent x1 connector board. It is a known issue for motherboards where each pcie socket is too close to the other one, like in the Asrock H110 Pro BTC+. I stopped having random gpus falling off the bus as soon as I covered each connector with isolating tape.