Curious behaviour after power cycle

I’ve got a Hue lamp and a couple of zigbee2mqtt USB dongles (same firmware June 2019) and with zigbee2mqtt 1.7.1 I find it all works. However if I turn off power to the Hue and then turn it on again I find I cannot communicate with it anymore. I can power cycle the dongle, restart the zigbee2mqtt but whatever I do it fails with a timeout error:

MQTT publish: topic 'zigbee2mqtt/bridge/log', payload `'{"type":"zigbee_publish_error","message":"Publish 'set' 'brightness' to 'hugh' failed: 'Error: Timeout - 1532 - 11 - 11 - 8 - 11 after 15000ms'","meta":{"friendly_name":"hugh"}}'`

The only way I can get it to work again is to swap the dongles, and then it all works. But same problem if I do it again. I have to swap the dongles again to get it started. It’s like there’s a memory of which dongle it was last using.

Is that possible? I’m quite puzzled by this. Any clues?