Service worker returning Offline page instead of 404 page when non-existant file is requested - progressive-web-apps

Question

I'm using this service worker for caching and offline mode.
when I am already on any existing page of my website and I request a non-existent page, the Offline page is served instead of the 404 page.
If, on the other hand, I close and reopen the browser and immediately request a non-existent page of my website, the 404 page is served correctly.
I suspect it has to do with these last few lines of the code
if(!matching || matching.status == 404) { return cache.match("/service/offline/"); but i don't know how to fix this problem. Can you help me please?
Here's the full service worker:
self.addEventListener("install", function(event) {
event.waitUntil(preLoad());
});
var preLoad = function(){
console.log("Installing web app");
return caches.open("offline").then(function(cache) {
console.log("caching index and important routes");
return cache.addAll([
'/assets/css/about.min.css',
'favicon.ico',
'/assets/js/script.js',
'manifest.webmanifest.webmanifest',
'/',
'/about/',
'/contact/',
'/service/offline/'
]);
});
};
self.addEventListener("fetch", function(event) {
event.respondWith(checkResponse(event.request).catch(function() {
return returnFromCache(event.request);
}));
event.waitUntil(addToCache(event.request));
});
var checkResponse = function(request){
return new Promise(function(fulfill, reject) {
fetch(request).then(function(response){
if(response.status !== 404) {
fulfill(response);
} else {
reject();
}
}, reject);
});
};
var addToCache = function(request){
return caches.open("offline").then(function (cache) {
return fetch(request).then(function (response) {
console.log(response.url + " was cached");
return cache.put(request, response);
});
});
};
var returnFromCache = function(request){
return caches.open("offline").then(function (cache) {
return cache.match(request).then(function (matching) {
if(!matching || matching.status == 404) {
return cache.match("/service/offline/");
} else {
return matching;
}
});
});
};