Reported by User:Oravrattas via https://www.wikidata.org/wiki/Wikidata:Report_a_technical_problem/WDQS_and_Search#Fewer_results_from_wdqs20*_than_wdqs10*
Running a simple query for all historic Dutch Senators consistently returns different results from all wdqs10* servers, than from all wdqs20* servers. I've been noticing odd results from this for many months, but it was remarkably difficult to track down, because it turns out that any queries I run locally only ever go to wdqs10*; the only way I have been able to replicate the behaviour is to run the query via Github Actions, which hits both sets of servers.
https://github.com/tmtmtmtm/ghatest2 shows this in action: the query in holders.js (including a run-time-specific comment to bypass caching) produces the outputs in results/ (named after the server handling the request and the date/time)
The results are consistent across the servers in each cluster: today each wdqs10* server returns 1361 rows, whereas each wdqs20* only returns 1357. These differences are actually made up of 8 people not being returned at all from wdqs20* (e.g. Q2053506 and Q2770742), 4 people who get returned twice from wdqs20* but only once from wdqs10* (e.g. Q2257961 and Q3469514), and one person who gets the same P39 data returned from each set of servers, but with a different schema:dateModified on each: (Q1845502, last modified on 2022-05-05 in the wdqs10* results, but on 2022-04-15 in the wdqs20* results).