Another thing you could try next time it happens is disable the firewall temporarily - then check again. If it still does not work then it’s probably safe to say it’s nothing to do with the firewall, if it begins working then I would be of the opinion I was in my previous post, that something might be reloading iptables.
The fact that curl still works suggests it could be something else though 
(and in which case you may need to post more details about how everything is set-up/deployed etc)
