please check SUP-119380
RouterOS 7.10 dns dynamic-servers random lost on all type device, RB5009/RB4011/RB450Gx4/CHR
Downgrading to version 7.9.2 everything is fine, but when upgrading to 7.10, random loss occurs.
this link
http://forum.mikrotik.com/t/7-10-stable-dns-crash/167135/1
maybe the same issue