not quite - as I currently don't have a ipv6 router with slaac enabled at hand - but I ran into an issue without one and the settings you saidPretty sure you can replicate it, (the dhcp server is from opnsense), if not i can still open a new thread? What do you think?
(it seems ifupdown2 still starts a dhcp client for ipv6 ... despite it not being written in the config) - I'll need to take a closer look and also see if this is the way in an environment with slaac advertisement.
do you have router-advertisements active in your environment?
In the meanwhile I'd suggest to stick with static IPs (or at least leave ipv6 on static) - you should be able to enable autoconf by the appropriate settings in sysctl.