To reproduce:

$version: "2"

namespace demo

use alloy#jsonUnknown

union MyOpenUnion {
  s1: Unit
  s2: Unit
  @jsonUnknown other: Document
}

Read this:

{"a": 42, "b": "oops"}

it'll all be persisted in the Document. I believe the behavior should be to fail decoding due to multiple root keys.

You can try it out in Smithy Transcoder: https://kubukoz.github.io/smithy-transcoder/#N4Igtg9gJgpgNgFRgDwC4gFwgCQDcYBOAzgJYQB2GABADogBMdN5z5AhmDEQA5sDGMKrEjNmAVyKC2cOBACeAYgBWRCgFVyAa3IQA7iwNjyZclQCycgPLcY5DSarBmVKkQCM1e6mev6n496mVAACKupaOvpUEKgAFoTUACIQfGKc5IEAviAANCAAZhAEYGzoGKBh5Jig+STwUABiJHCohJggiQCiDQCCagAyCCCZmXkk5NxiZSBOIGx01AAs9Dm0IABGC2sQENxEdNl5+MQmmG6ZQA