Fix incorrect behavior in lenient tagged union decoders

PR Checklist (not all items are relevant to all PRs)

fixes #1617

[x] Added unit-tests (for runtime code)
[ ] Added bootstrapped code + smoke tests (when the rendering logic is modified)
[ ] Added build-plugins integration tests (when reflection loading is required at codegen-time)
[ ] Added alloy compliance tests (when simpleRestJson protocol behaviour is expanded/updated)
[ ] Updated dynamic module to match generated-code behaviour
[ ] Added documentation
[x] Updated changelog