Expansion failure, new since .Net 5 upgrade

So I have a custom Action on one of my Odata controllers where I post a set of data of type T and I get back a set of results of type Result

Essentially what we do is something like this ...

HTTP POST  odata/T/UpdateAll?$expand=Item($expand=References,Buckets,Companies,Lines)
Body: 
{
    "value": [  array of T ]
}

Pushed to this action method ...

[HttpPut]
[EnableQuery()]
public virtual async Task<IActionResult> UpdateAll([FromBody] ODataCollection<T> items, ODataQueryOptions<Result<T>> queryOptions)
      => ModelState.IsValid ? Ok(await Service.UpdateAllAsync(items.Value)) : BadRequest(ModelState);

The controller hands of the business logic work to a dependency and is then given back a IEnumerable<Result> to return to the caller.

The call above does this work then wants back "for each result" it's Item property then expand in to the items child "References,Buckets,Lines" properties.
This used to work fine under .Net Core but since our .Net 5 upgrade the expand is returning the items but not the second level expand.

Is there something I am missing here / some key difference in the way the Odata 8.0.1 package or .Net 5 implementation works that prevents this?