A few things:

That's a consequence of the "Anytime Query" feature in Virtuoso that provides partial solutions in situations where a query cannot be completed within a specific timeframe. This timeframe takes the form of a configurable timeout, and is an critical feature for enabling global ad-hoc query access, 24/7, 365 days a year re the likes of DBpedia and Wikidata.

For the record.

In T206561#7304519, @So9q wrote:

I took a glance at Virtuoso.

I found nothing about scaling Virtuoso to a cluster (which is IMO what WMF needs because of growing amounts of data and reaching the limits of what 1 machine can handle)

A snippet from WP:
"Virtuoso is designed to take advantage of operating system threading support and multiple CPUs. It consists of a single process with an adjustable pool of threads shared between clients. Multiple threads may work on a single index tree with minimal interference with each other. One cache of database pages is shared among all threads and old dirty pages are written back to disk as a background process."

Virtuoso IMO is not the way forward for WMF. We need a distributed graph/column database with SPARQL on top. See https://phabricator.wikimedia.org/T289561 for an application that has exactly that (but seems abandoned since dec 2020 unfortunately)