This PR #44104 introduces a breaking change that is only experienced at runtime.
ResourceSet implements IEnumerable and ASP.NET Core is using the contract.
At least there, but other tests break so it might not be isolated.
https://github.com/dotnet/aspnetcore/blob/master/src/Localization/Localization/src/Internal/ResourceManagerStringProvider.cs#L75
While we could react to this change, it doesn't seem that this change of behavior was intentional.