`Microsoft.Authorization/roleAssignments` declares support for `2020-10-01-preview`, but does not work with ARM

Reported here: https://github.com/Azure/bicep/issues/784#issuecomment-896046402

Here it's declared in swagger spec: https://github.com/Azure/azure-rest-api-specs/blob/88e7838a09868a51de3894114355c75929847a46/specification/authorization/resource-manager/Microsoft.Authorization/preview/2020-10-01-preview/authorization-RoleAssignmentsCalls.json#L231

However there is no [manifest configuration](https://armmanifest.visualstudio.com/One/_git/Manifest?path=%2FMICROSOFT.AUTHORIZATION%2FPROD%2FMANIFEST.JSON&version=GCcedbd3983eda2f5d0bc1f96b6258cf5d7603715a&line=464&lineEnd=484&lineStartColumn=1&lineEndColumn=13&lineStyle=plain&_a=contents) for this API version.

Start by comparing the 2020-10-01-preview declaration in specification/authorization/resource-manager/Microsoft.Authorization/preview/2020-10-01-preview/authorization-RoleAssignmentsCalls.json with the linked MICROSOFT.AUTHORIZATION/PROD/MANIFEST.JSON configuration. Verify the reported ARM behavior and determine whether the API version is consistently represented. Done means the specification and ARM manifest agree and the version works as declared.

Written by the indexing model from the issue text.