Dispose sometimes called after a while (or never) when tab or browser closed

I have simple app with one page that can be access only by one user at a time. I've made Singletone object that tracks who currently view a page. When user close tab, browser lock is released - in Dispose method i remove lock in singletone object. Problem is that sometimes when i close a tab or browser, dispose is not called instantly but after couple of minutes and sometimes never (after 2 days). I figured out that when i close browser in Firefox, dispose is never called. Is this lag by design? Is there better way to solve this problem?

I call dispose like this:

Edit: I'm using .NET 6

void IDisposable.Dispose() 
{ 
    base.Dispose(); 
    //AppData is singletone object 
    AppData.InUseByuser=null; 
}