Your valve has an anti-scald feature which automatically adjusts the water temp if cold water is diverted to another location, e.g., a flushed toilet, washing machine filling, etc. Most valves have an adjustment screw to allow you to set the temp. Since each manufacturer is different you'll need the manual or contact the manufacturers support. BTW, these anti-scald valves are being added to building codes in many parts of the U.S.