[Samba] Samba 4.7.4 + bind9 DLZ /backend/ dropping delegated domain

[Zdravko Zdravkov]

Here's the problem. I've got working samba AD server. I've configured it to
work with bind9 DLZ backend and I'm managing the DNS entries from the
windows DNS manager.
I've made delegated DNS record for our EMC cluster storage, according to a
guide. The record points to the SmartConnect service IP of the EMC.
Since last week I've noticed that my DNS queries to the storage aren't
being resolved which is a problem because that way the storage isn't
accessible. Every other local records are resolving and there's no problem
with them.
Rebooting the whole server seems to fix this for some time. This morning I
found out the problem reoccurred again though.
Could you point me to some steps towards debugging this? ANy help will be
appreciated.