[EF Core / DbContext] If I .Add() a row in a seperate thread, related tables/rows' state are set to EntityState.Added => Exception

Just to make it clear:

I use an injected instance of DbContext when it runs outside the thread, and a new DbContext() inside the thread.

Whether I run the code in a seperate thread or not, the ChangeTracker has Entries for the row I want to add, plus rows from all related tables.

The difference is that when I try to add a row in a seperate thread, all the related rows' EntityState are set to EntityState.Added:

db.VehicleWarehouse.Add(vehicleWarehouse);

entries = db.ChangeTracker.Entries();
cnt = entries.Count();

if (cnt > 1)
{
    foreach (var entry in entries)
    {
        var entity = entry.Entity;
        var state = entry.State;

        if (entity != vehicleWarehouse)
        {
            // all related tables ends up in a seperate entity, up to the very top (VehicleWarehouse->Vehicle->Company)

            // if the code is in a seperate thread, State is always set to Added => Exception
            if (state == EntityState.Added) 
                db.Entry(entity).State = EntityState.Unchanged;  // prevent insert exception
        }
    }
}

db.SaveChanges();

I start the thread like this:

Task.Run(() => {
    FindVehicleWarehouse(vehicle, isLooking);
});

What could be causing this, and is there a way to prevent it from happening?