That's right @Barrick. The implementation of our query providers is not exactly the same as the standard LINQ. So, Stephan, that's the issue.
If you just want to get specific fields I would suggest you to use IDSQuery like:
QueryService<Account> AccQueryService22 = new QueryService<Account>(context);
var t13 = AccQueryService22.ExecuteIdsQuery("Select Id, Name From Account Where Active in (true, false)");
I will forward the feedback to our team.
Thanks!