fix memory leak #1506

[mhornbacher]

Description

This bugfix is a PR for @WayneHiller's find of a memory leak in async calls. Verified independently on .NET Core 3.1

I am making the PR to be the one available to handle issues related to getting this bugfix into an upcoming release (it seems there has been some issue with the versioning)

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Ensured that existing tests cover this logic. Sadly due to the nature of the bug and the .NET runtimes, easily repeatable tests are not possible to write (memory constraints on test server play a large part)
• I have added necessary documentation (if appropriate)