Interesting that em0 would have a IPv4 problem, since that's been problem free. Is miniupnpd strictly UPnP? If so, it's possible that's not working, it's not something we use very often.
Yes, that's for UPnP (and NAT-PMP) only, so probably doesn't matter for you. Could just be a timing-related issue.
And suggestions on what to look at for this?
I'd fire up packet captures on your pfSense box's WAN and LAN interface and then try to ping an external address from a LAN client. I'm assuming you do have a firewall rule that allows IPv6 traffic from "LAN network" (as opposed to something hard-coded)?