systemd version the issue has been seen with

247, 247.1

Used distribution

NixOS

Linux kernel version used (uname -a)

Linux 5.4.83

CPU architecture issue was seen on

x86_64

Expected behaviour you didn't see

networkd honors the net.ipv6.conf.$interface.use_tempaddr sysctl values set when IPv6PrivacyExtensions=kernel is set

Unexpected behaviour you saw

net.ipv6.conf.$interface.use_tempaddr is -1, no IPv6 private IP addresses are assigned, traffic goes out with the EUI-64 address

Steps to reproduce the problem

Configure your interface with IPv6PrivacyExtensions=kernel, set the net.ipv6.conf.$interface.use_tempaddr sysctl to 2`, plug it into a network with IPv6 router advertisements

Bisecting showed this seems to have been introduced by 5e0534f, which was part of #17240. More details are in NixOS/nixpkgs#106858 (comment).