Our workaround was to disable subscription persistence so that Asterisk forgets them on startup and requires the phone to re-request them. We did this by modifying our sorcery.conf file with the following:
[res_pjsip_pubsub]
subscription_persistence=memory
Again, this is 15.x and we are not using FreePBX. I believe the setting should still apply but I’m not familiar with 13.x and don’t know how easy it is to get to the Asterisk configs with FreePBX. It’s something to try if you get a chance, though.