Ajax REST Tooling Queries Return Null SymbolTable?

Question

Background

I'm trying to build upon Daniel Peter's approach in his HyperBatch library to build a HyperQuery tool that will allow me to get better insight into some large tables we have without needing to export the whole table to do external analysis.

Problem

Unfortunately, I stalled out at a pretty early stage, as I'm having trouble getting any Javascript code to identify which classes extend an interface. That's because every single class is returning a null SymbolTable.

Analysis

• I simplified the AJAX as far as I could and still I get SymbolTable = null for every single ApexClass.
• The below MVR works just fine in other sandbox environments, so it seems like the problem is specific to this sandbox.
• The exact same query in the developer workbench returns a populated SymbolTable, so the problem does not appear to be my class(es) or profile.
• The SymbolTable is null for every ApexClass record, even after I recompiled all classes.
• If I run the query via Execute Anonymous, the SymbolTable is populated. I can pass that same session id into my AJAX call instead of what I get from the GETSESSIONID merge variable and the behavior remains unchanged.

Question

Why might my SymbolTable values be null only when queried via Javascript?

Repro

Javascript

Page/Script

<apex:page showHeader="false" sidebar="false">
    <script>
        "use strict";
        var query = "SELECT Name, SymbolTable FROM ApexClass WHERE NamespacePrefix = null",
            url = "/services/data/v39.0/tooling/query/?q=" + encodeURIComponent(query),
            request = new XMLHttpRequest();

        request.open("GET", url, /*async*/ true);
        request.setRequestHeader("Authorization", "Bearer {!GETSESSIONID()}");
        request.onreadystatechange = function () {
            if (request.readyState === 0 || request.readyState === 4) {
                var i, records = JSON.parse(request.responseText).records;

                console.debug(records); // results show up
                for (i = 0; i < records.length; i++) {
                    if (records[i].SymbolTable) {
                        console.debug(records[i]); // nada
                    }
                }
            }
        }
        request.send();
    </script>
</apex:page>

Console Log

Workbench

Endpoint

/services/data/v39.0/tooling/query/?q=SELECT+SymbolTable+FROM+ApexClass+WHERE+Name='HyperQuery'

Result

Execute Anonymous

Script

String query = 'SELECT+Name,SymbolTable+FROM+ApexClass+WHERE+Name=\'HyperQuery\'';
String root = URL.getSalesforceBaseUrl().toExternalForm();
String endpoint = root + '/services/data/v39.0/tooling/query/?q=' + query;

HttpRequest request = new HttpRequest();
request.setHeader('Authorization', 'Bearer ' + UserInfo.getSessionId());
request.setEndpoint(endpoint);
request.setMethod('GET');

Map<String, Object> response = (Map<String, Object>)
    JSON.deserializeUntyped(
        new Http().send(request).getBody()
    );
List<Object> records = (List<Object>)response.get('records');
Map<String, Object> result = (Map<String, Object>)records[0];
system.assertNotEquals(null, result.get('SymbolTable'));
system.debug(result.get('SymbolTable'));

Debug Excerpt

{constructors=({annotations=(), location={column=5, line=6}, modifiers=(private) ...