I have this function in a controller:

[ServiceFilter(typeof(LoggedInAttribute))]
[HttpPut("bio")]
public ActionResult<string> ChangeBio(string UserId, string Bio)
{ 
    // ... 
}

And this attribute class:

public class LoggedInAttribute : ActionFilterAttribute
{
    // ...

    public async override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        // Here the `filterContext.ActionArguments.Keys.Count` is always zero
        // no matter what action it's called from
    }

    // ...
}

Why could this be happening?

This puzzles me the most, in one field it sees the parameters and in the other it doesn’t?