As anyone who bothers to read my comments (BTW "hi" to both of you) already knows, I despise systemd with a passion, but this one is more an IPv6 problem in general.
Yes this is an actual bug in networkd, but IPv6 seems to be far more bug prone than v4, and problems are rife in all implementations. Whether that's because the spec itself is flawed, or because nobody understands v6 well enough to implement it correctly, or possibly because there's just zero interest in making any real effort, I don't know, but it's a fact nonetheless, and my primary reason for disabling it wherever I find it. Which of course contributes to the "zero interest" problem that perpetuates v6's bug prone condition, ad nauseam.
IPv6 is just one of those tech pariahs that everyone loves to hate, much like systemd, albeit fully deserved IMO.
Oh yeah, and here's the obligatory "systemd sucks". Personally I always assumed the "d" stood for "destroyer". I believe the "IP" in "IPv6" stands for "Idiot Protocol".