Tooling REST API CalloutException Read timed out

We’ve got an issue regarding Salesforce improperly cleaning out FlowElementTestCoverage records, leaving over a million "stranded" on an old/inactive Flow Version. This is causing us to be unable to delete said Flow Version.

The object in question can only be queried/deleted via the Tooling API, and I’ve written the following just to test retrieval:

HttpRequest req = new HttpRequest();
req.setMethod('GET');
req.setHeader('Authorization','Bearer '+accessToken);

String urlQuery = EncodingUtil.urlEncode('SELECT Id, FlowVersionId FROM FlowElementTestCoverage WHERE FlowVersionId = '[myFlowVersionId]' LIMIT 1', 'UTF-8');
req.setEndpoint('https://[domain].my.salesforce.com/services/data/v49.0/tooling/query/?q='+urlQuery);

HttpResponse res = new Http().send(req);
System.debug(res.getStatusCode());
System.debug(res.getStatus());

Problem is, I always get System.CalloutException: Read timed out. However, when I remove the WHERE clause such that the query is SELECT Id, FlowVersionId FROM FlowElementTestCoverage LIMIT 1, it works fine.

I guess I’m just a little lost as to how to proceed. I don’t understand how adding a FlowVersionId filter would cause this problem.

SELECT Id,FlowVersionId FROM FlowElementTestCoverage WHERE FlowVersionId='[myFlowVersionId]' LIMIT 1'