I've got an AC-86U.
With the upgrade to 384.8, it's happened to me twice now that dnsmasq stops sending out IPv6 router advertisements. It happened once after the upgrade to 384.8, and once after the upgrade to 384.8_2. When it happens, dns resolution still seems to work, and dnsmasq responds to router solicitations, but it doesn't send out the unsolicited RAs every 10 seconds like it's supposed to. As a result my machine loses its IPv6 address after 10 minutes.
A reboot of the router resolves the issue. So far, no discernible pattern as to when it happens.
It's been rock-solid for years before 384.8, so I suspect a bug in the new dnsmasq.