Hi there
Just registered to post my conclusion.
I had the same issue with my pfsense box v. 2.3.3.
Unbound-service was restarting very frequently, unbound worked, but due to the restart i had sometimes gaps where in some special cases services broke down / where not able to synchronize etc.
It was caused by dhcp which tried to register new leases or entries (not 100% what it exactly wanted to register) but i see a lot of dhcpv6 log-entries.
At the end i figured out that on my WAN interface that the configuration type of ipv6 was dhcp, after i switched it to none, all problems disappeared.
Why this configuration exactly caused this problem, not sure at this moment, i'm anyway not using actively ipv6.
I will post more information if i have them.
Hopefully this helps.
Cheers, treeol