So one of the main features of dhcpcd was the ability to add and remove addresses and subnets in accordance with interface preference and state. This worked very well in Linux, both for IPv4 and IPv6.
When I discovered FreeBSD and then NetBSD making dhcpcd work with the same features provided troublesome. For the IPv4 case, we needed to patch the kernel so that [IFA_ROUTE] 1 [remained sane] 2. For the IPv6 case it was a lot more complicated as parts of the IPv6 stack rely on the kernel processing Router Advertisements instead of a 3rd party tool. Working this out was a lot more time consuming and complicated, [but we got there] 3 [with minimal fuss] 4!
So as of now, dhcpcd can fully replace the NetBSD and Linux in kernel Router Advertisement handling code.
But why was I right after all these years? Because quite a few knowledgeable NetBSD folk assured me that the kernel was fine and it was likely dhcpcd at fault. As it turned out, it really was the kernels fault.