Rockpi4a MAC addresses not unique

My cluster consists of 18 rockpi4a boards booting the latest ubuntu image from nvme ssds (rockpi4a_ubuntu_focal_server_arm64_20210125_2342-gpt.img).
i was surprised that only several boards have unique MAC address and there are many duplicities. It is strange since the MAC reported by u-boot is correct but it changes after the ubuntu boots up.
Is there a way how to solve it? It seems like an OS bug to me…

This is the nmap output from after connecting all the nodes: https://pastebin.ubuntu.com/p/ZD5JBH9Wsd/