Firefox localstorage values sometimes missing in new tab

I am facing a strange issue in Firefox (62.0.3). The values set in localStorage are sometimes lost in a new tab.

I can reproduce this with the following HTML, when served by a web server (I used node/express).

<!DOCTYPE html>
<html><body><script>
    window.onload = () => {console.log(window.localStorage.myVal || 'undefined')}
</script></body></html>

Steps:

1. open page, page displays undefined
2. run script in console localStorage.myVal='defined'
3. reload page, page displays defined
4. when opening the page in a new tab it displays undefined or defined at random. (I opened 10 new tabs and results seemed random)

Maybe I have set something in Firefox which causes this, but since the behaviour is at random I am somewhat puzzled.

The example above is just to demonstrate the problem. The Actual application where this problem first happened was a react/redux application using localStorage.