Copilot assigned issues breaks `add-milestone`

Tried using add-milestone for 7.9 NuGet release and one issue failed to update.
Sounds like the response is indicating a problem with assignee of Copilot.
https://github.com/NuGet/Home/issues/14939

Failed updating https://github.com/NuGet/Home/issues/14939.
Octokit.ApiValidationException: Validation Failed
at Octokit.Connection.HandleErrors(IResponse response) in //Octokit/Http/Connection.cs:line 785
at Octokit.Connection.RunRequest(IRequest request, CancellationToken cancellationToken, Func2 preprocessResponseBody) in /_/Octokit/Http/Connection.cs:line 766 at Octokit.Connection.Run[T](IRequest request, CancellationToken cancellationToken, Func2 preprocessResponseBody) in //Octokit/Http/Connection.cs:line 751
at Octokit.ApiConnection.Patch[T](Uri uri, Object data)
at NuGetReleaseTool.AddMilestoneCommand.AddMilestoneCommand.AddMilestoneToIssueAsync(Milestone expectedMilestone, Issue issue) in C:\apps\Entropy\NuGetReleaseTool\NuGetReleaseTool\AddMilestoneCommand\AddMilestoneCommand.cs:line 93
{"message":"Validation Failed","errors":[{"value":"Copilot","resource":"Issue","field":"assignees","code":"invalid"}

Start in NuGetReleaseTool/AddMilestoneCommand/AddMilestoneCommand.cs at line 93, where AddMilestoneToIssueAsync updates the issue. Reproduce the failure with NuGet/Home issue 14939 and inspect how its Copilot assignee is handled. Done means add-milestone can process that issue without the Octokit validation error.