Strange behaviour in JavaScript single page application

I am developing a single page application using Vanilla Js. My code contains a function which utilizes fetch using a proxy to get data from a site. But, the problem is that after I open the single page application (index.html file) and come back to it later it redirects to the website with the url specified in fetch. For example, if this is my Js function

...
function getData() {
    fetch('https://young-waters-99383.herokuapp.com/https://example.com/thisFile.md')
    .then(response => response.blob())
    .then(blob => blob.text())
    .then(data => {
        let myDiv = document.querySelector('#myDiv');
        myDiv.innerHTML = data;
    }
}
...    

then the website now goes to https://example.com/thisFile.md after I leave it open and return to it after some time. What could be the problem?