var page: IXQValue;
begin
page := xqvalue('http://192.168.1.5/xxxx/test.html').retrieve();
writeln(page.map('form(//form,{"5":"1","6":"1","7":"2","8":"1","9":"1","button":"set"})').debugAsStringWithTypeAnnotation());
writeln(page.map('form(//form,{"6":"1","7":"2","8":"1","9":"1","button":"set"})').debugAsStringWithTypeAnnotation());
gives
object: {method: string: GET, url: string: http://192.168.1.5/test.html?5=1&6=1&7=2&8=1&9=1&button=set}
object: {method: string: GET, url: string: http://192.168.1.5/test.html?5=2&6=1&7=2&8=1&9=1&button=set}
as it should.
Do you get something else?