I’m not sure exactly what you’re expecting to happen, so I can only speak in generalities.
Anything you write in a lifecycle event handler should perfectly fine with being executed only once, or many times. The only thing you should rely on is that with paired ones (such as ngOnInit / ngOnDestroy), they’ll be paired. You won’t get two calls to ngOnInit without an intervening ngOnDestroy.
So, if you’re going to wire up sources of data in ngOnInit, they have to be capable of responding to changes if that can happen. Sometimes you don’t care about this. This is apparently not one of those times. So, as I see it you have two choices: move what is in ngOnInit now out of it, or don’t use snapshots in there - subscribe to long-lived Observables instead, as described in here.