The autofill valve may be misadjusted, or it is leaking. Another possibility is if there is a leak in the heat exchanger of the indirect water heater, but it may go higher than the 25psi that you're seeing. Either the expansion tank is not sized properly or there's a valve between it and the rest of the system, is another possibility. Once you have your 12psi, shutoff the valve (hopefully, they installed one) feeding the autofill valve and see if it stays stable. If so, you've identified the autofill valve as the issue.