That call happens here in the code on the window unload event and is used to tell the signalr server that you are disconnecting.

If you are seeing a lot of these it may be that your proxy is not configured correctly for websocket connections.

window.addEventListener(
    'unload',
    () => {
      const data = new FormData();
      const circuitId = circuit.circuitId!;
      data.append('circuitId', circuitId);
      navigator.sendBeacon('_blazor/disconnect', data);
    },
    false
  );

There's a bug reported for this issue here: https://github.com/dotnet/aspnetcore/issues/30316

Blazor.server.js not respecting configureSignalR builder defined url/path for disconnnects

As a work-around, I added this to Startup.cs in the Configure method:

var rewriteOptions = new RewriteOptions();
rewriteOptions.AddRewrite("_blazor/disconnect", "/_blazor/disconnect", skipRemainingRules: true);
app.UseRewriter(rewriteOptions);