When a benchmark returns a ValueTask, it is simply called with .GetAwaiter().GetResult().

This is illegal ValueTask usage: you must not call GetResult() until the task has been completed.

IValueTaskSource-backed tasks do not need to support blocking on GetResult(). The in-box ManualResetValueTaskSourceCore does not support blocking GetResult(). Nor does the DOTNET_SYSTEM_THREADING_POOLASYNCVALUETASKS feature in .NET 5.

Instead, the ValueTask should be converted to a Task first:

static void BlockForResult(ValueTask task)
{
    if (task.IsCompleted)
        task.GetAwaiter().GetResult();
    else
        task.AsTask().GetAwaiter().GetResult();
}

(This will be unfortunate for the memory diagnoser, as a bogus Task will show up, but the alternative is to just crash...)