After upgrading from 12 to 13 I also had problems with Polycom 301, 501 and 550’s unregistering for no reason the registering again. I saw this post :-
and I wondered if it might have some bearing. When I upgraded to 13 asterisk went from 13.5 to 13.6, my other version 13 (clean installs not upgraded from 12) were still on 13.5
I downgraded the asterisk version to 13.5 and so far no more problems over the last 5 days.