An errata patch for icmp6_linklocal has been released for OpenBSD 6.1. When pinging an IPv6 link-local address, the reflected packet had ::1 as source address. The echo reply was ignored as it must be from the link-local address.
Binary updates for the amd64 and i386 platforms are available via the syspatch utility. A source code patch can be found on the respective errata page: https://www.openbsd.org/errata61.html As this affects the kernel, a reboot will be needed after patching. If IPv6 is not configured on your system, this issue does not affect you.