Short Answer: You should use event.waitUntil and pass a promise to it, which returns showNotification eventually. (if you have any other nested promises, you should also return them.)
I was expriencing the same issue but after a long research I got to know that this is because delay happen between PUSH event and self.registration.showNotification(). I only missed return keyword before self.registration.showNotification()`
So you need to implement following code structure to get notification:
var APILINK = "https://xxxx.com";
self.addEventListener('push', function(event) {
event.waitUntil(
fetch(APILINK).then(function(response) {
return response.json().then(function(data) {
console.log(data);
var title = data.title;
var body = data.message;
var icon = data.image;
var tag = 'temp-tag';
var urlOpen = data.URL;
return self.registration.showNotification(title, {
body: body,
icon: icon,
tag: tag
})
});
})
);
});
Minimal senario:
self.addEventListener('push', event => {
const data = event.data.json();
event.waitUntil(
// in here we pass showNotification, but if you pass a promise, like fetch,
// then you should return showNotification inside of it. like above example.
self.registration.showNotification(data.title, {
body: data.content
})
);
});