Since I've been running Gentoo on my x250, I had one peculiar problem: upon resuming from suspend, I'd lose IPv6 connectivity. In short, the only way to 'fix' it, was toggling the radio. Not quite what you want.
After installing Gentoo on my new work machine, I noticed restarting systemd-networkd gave the exact same troubles. Convinced I should now finally fix this, I fired up tcpdump one more time and noticed no Router Solicitation was sent out.
With the proper search terms, that lead me to this three year old issue, with a comment from user wonderfly:
wonderfly pointed out enabling the kernel configuration
had fixed this issue for him. I'm happy to say that after enabling it, it fixed
the issue on my laptop as well as my new workstation.