Leading Zeroes on Arduino Ethernet MAC Address
Writing this in case someone in the future might be interested.
The values passed in the Ethernet library is correct. The last value, 0x01 must be 1 and is written correctly, 0x01 in the registers of the W5500 inside the Ethernet shield.
Where I've got it wrong is how I pass the MAC address to the server. I'm basically creating a string for a json message. In there, I've put %02x to make sure that zeroes are padded.