They all do that. It is quite normal. After calibration, Pmax appears on the screen.
Calibration mode can be forced in peak mode at any time, by holding the 'Peak' button for more than 2 seconds (2 beeps).
'Entering Peak mode will automatically execute peak calibration, if peak calibration was never done before. The Calibration
process measures and stores the offset voltage of internal operation amplifiers. High precision is achieved by substracting this offset from reading. When power off the ES51922 or switch the measurement mode to °F temperature mode, the calibrated
offset value will be cleared. ES51922 will automatically execute peak calibration, if peak mode was activated again after the calibrated offset value was cleared.'