ParamValidator fails to properly validate document type trait

[JavierLuna]

Confirm by changing [ ] to [x] below to ensure that it's a bug:

• I've gone through Developer Guide and API reference
• I've checked AWS Forums and StackOverflow for answers
• I've searched for previous similar issues and didn't find any solution
• This is an issue with version 2.x of the SDK

Describe the bug
I'm using Smithy's document type trait to generate an SDK. The generation itself goes well, but when attempting to use it to call my service, I get an UnexpectedParameter: Unexpected key 'test' found in params.myDocumentType.

The validation error appears to come from ParamValidator, I'm guessing #3808 did not adapt parameter validation.

Disabling paramValidation works: The SDK does the call and the service processes it correctly.

Is the issue in the browser/Node.js?
Node.js

If on Node.js, are you running this on AWS Lambda?
Local

Details of the browser/Node.js version

$ node -v
v14.17.0

SDK version number
Example: v2.1007.0

To Reproduce (observed behavior)
Attempt to make any call to a service that has a property with the document type trait.

Expected behavior
Parameter validation correctly validates my document property and makes the call to my service.

Screenshots
No

Additional context
No