I’ve added a few Mi Smart Plug zigbee devices with no problems. Just kept the button pressed for 5s on the Smart Plug and the devices appeared in Configuration > Devices section.
But I’ve removed one of Smart Plug device (deleted it from Devices section in HA) and now I can’t add it back.
This removed Plug has the LED turned blue when I start it.
I can also see it in with MQTT Explorer under /zigbee2mqtt/ tree node.
But it does not appear in MQTT Explorer under /homeassistant/switch/ tree node, like the devices that are add in HA.
So I guess this device does communicate with MQTT, but it’s just “blacklisted” in Home Assistant (becase I’ve removed).
Syncing Zigbee again by long pressing the button of this deleted Plug does not help: it just won’t appear in HA interface under Devices.