I think you’re right (I even remember I stumbled upon that page but discarded on the spot given the Nitro reference). The relevant part is
By default, the IPv6 preferred lease time is 140 seconds and DHCP lease renewals for IPv6 address assignments occur at the halfway mark of 70 seconds
therefore it makes sense seeing logs around every minute or so (precisely, every 70 seconds).
This is for sure something not clear from AWS console VPC -> DHCP option sets where no such default 140s default lease time is reported and cannot be changed at all in the existing DHCP config set (no edit button at all):
but you can set the IPv6 DHCP lease time for a brand new DHCP config set!
Due to this, I think we can consider closed the issue!