While going through the Telegram history of the Wikibase community channel for the WBUG 2020 report I saw that on 11 June 2020 the issue of prefixes and URIs for the query service was raised. Simple queries like wd:Q1 wdt:P1 ?item were expected to work.
It was suggested that the full URI needed to be used, e.g. https://tinyurl.com/y9vakc2c, or an equivalent prefix, and that further information was in https://wiki.wbstack.com/wiki/Query_Service#URIs_and_prefixes - this document is no longer accessible, and the current docs site does not appear to contain equivalent documentation. (It was a point of confusion for me, too.)
https://wbstack.github.io/docs/users/platform.html also states that https://wiki.wbstack.com contains legacy docs.