wyupdate.exe w/ .NET 4.0 failing to decrypt update (0x63 error)

We are unable to install encrypted updates using wyupdate.exe built with .NET 4.0. We are fairly confident this was working fine before. Not super clear on when this started happening. (did a Windows update break this?)

Repro steps notes:-client/update generated with same .wyp-wyUpdate files built for .NET 4.0-Update encrypted-Problem occurs independently if whether update signed or not-Using Windows 10.0.16299-Easy to reproduce with dummy update-Works perfectly fine if wyUpdate built for .NET 2.0-Not encrypting the update resolves the issue (obviously)

Is this a configuration issue or a bug?