I've had the same issue on the AC66U in a mixed network. I'm wondering if it's related to one of these lines in /etc/dnsmasq.conf:
dhcp-option=lan,6,208.67.222.222,208.67.220.220,0.0.0.0
dhcp-option=lan,252,"\n"
Shouldn't the 0.0.0.0 be 192.168.1.1? Also, a newline character seems odd to me...