Received thru the DSS-X public comments list from Neil Crossley on the 2019-08-23:

The SimpleContent declaration is consistently mapped to an optional property in the JSON schema. Strictly speaking, this leads to a different semantic expressiveness in the resulting JSON schema. Consider a valid XML instance of X509DigestType - it's content is always present, but possible empty. However, a JSON object instance of dss2-X509DigestType may have no value property, or it's property 'value' may contain the empty string. Both JSON objects are valid but not necessarily or obviously equivalent.