The photo doesn't really help that much without knowing specifically what each of the wires in the photo is connected to at their other end.
Unfortunately, there is not set format for the wiring colours or cables used; they would have been chosen by the original installer based on their personal preferences and the various components used (ie thermostats & boilers) don't all have exactly the same connections.
However, some things are identifiable. In the photo below
The two brown wires in the bottom of the terminals circled are the switched lives going to the two motorised valves (one for each zone) and the black wires with the brown sleeves above, that are in the same terminals should be the switched lives from the two room thermostats. So it looks as I would expect. The question is: Do the two black wires go to the 'switched live' of the appropriate room thermostat as they should, and from the photo it is impossible to tell.
As it's a new build house the recourse should be for the builder to rectify it. If you try to DIY fix it and get it wrong, or damage a component they will be unlikely to honour any warranty.
It is of course possible, albeit more unlikely, that the motorised valves could be wired and working as they should be, but they have been incorrectly installed into the plumbing.
If you operate the room thermostats one at a time and have someone listening at the motorised valves you should be able to hear a whirring noise as they close when the thermostat switches off. If they both 'whirr' when the upstairs thermostat goes off they are indeed incorrectly wired. [If you do this test, allow 30 seconds for the valve to fully wind open first before turning the thermostat down]
From the test, should you find that one room thermostat operates one valve, and the second thermostat operates the other valve then the wiring would appear OK.