You can add me to the list of people with issues on 2.1.5 with the dnsmasq process eating 100% of a cpu thread and spotty dns functionality from clients on lan/vlan10/vlan20 dhcp'd interfaces. I have 2 Soekris net6501 boxes here on 2.1.5 -
I used my spare box to test an upgrade to 2.1.5 awhile back and left it in production with the primary sitting off to the side for a month or so. I then took the primary and put it back into play, ran the upgrade, then imported the backed up config from the backup box. I added 2 vlan interfaces to my lan interface and configured the firewall rules for the new segments and got all of that working as expected. The following morning the issues started up (or I should say users started seeing them).
At this point if there was some kind of fix in 2.2, I suppose its a debate of rolling back to 2.1.4, waiting for a 2.1.6 (if they release one with a patch for this), taking my chances on 2.2BETA in production, or waiting for 2.2 release.