Not sure if this is related to this particular case. From previous experience, PayPal will silently stop delivering IPNs if your webhook URL ever returns a non-200 status code for an extended period of time.
It took us a fair amount of time to figure out that this was the cause of our issue -- the PayPal UI didn't indicate any problems and PayPal business customer support was unable to see the source of the problem either.
It took us a fair amount of time to figure out that this was the cause of our issue -- the PayPal UI didn't indicate any problems and PayPal business customer support was unable to see the source of the problem either.