need help with tachometer signal

i have connected an PROTECHNIC MGT4012WB-W28 PWM 40MM FAN to my vcore 3 an it works flawless with alittle issue that maybe occours in othrt setups as well till 50% fan speed the readings are normal but when i go higher the shown rpm are going down instead of up as shown in the pictures strange behavior. pleasee help me with your knowledge.