Blazor Server - DbContext is disposed after refreshing page

I am developing a Blazor server app, where all clients will have a list of things, any one of these clients can update thing which should then trigger a callback telling all clients to call DbContext.Entry(thing).Reload() so they're up to date. This works all great and well until I refresh the page, then I get the Cannot access a disposed object error and I can't figure out how to get around it.

I have the follow services:

services.AddDbContextPool<MainDbContext>(...);
services.AddSingleton<RefreshService>();

RefreshService.cs:

public class RefreshService {
    public Func<long, Task> OnRefreshThing { get; set; }

    public void RefreshThing(long thingId) => OnRefreshThing?.Invoke(thingId);
}

Index.blazor:

protected override void OnInitialized() {
    RefreshService.OnRefreshIssue += OnRefreshIssue;
}
private async Task OnRefreshThing(long thingId) {
    // This works perfectly until I refresh the page & try to call it again
    Thing thing = await MainDbContext.Things.FindAsync(thingId); // exception is thrown here

    await MainDbContext.Entry(thing).ReloadAsync();
}

And here's an example of what triggers the error:

Thing thing = Things.Where(t => t.ThingId == 1);
thing.Name = "New name";
RefreshService.RefreshThing(thing.ThingId);