Zigbee is a mesh network, but Zigbee with mqtt has a hub that stores messages. I haven't used it myself but it would mean that if, say, a Zigbee bulb was routing a message on the mesh network through a smart switch across the room to the hub, and the switch dropped the connection for a moment, a hub reply could be dropped entirely. Just briefly, but thatd be the intermittent issues that people are describing here.

MQTT stores all those messages in the hub though and makes the light bulb check in to get the messages, so if a light bulb were to do that and the switch disconnected, the light bulb would notice the failure and just retry, and the message is still on the mqtt hub to be redelivered.

Dunno if this description is exactly correct, but it sounds like it from my brief look on Wikipedia on communication differences.