How are you measuring the power out of the 7610 -with its internal wattmeter? If you are in WSJTX - the "tune" is a CW carrier at same level as the FT8 signal. See what power is in"tune" ,then use "Enable Tx and set Tx even/first so it goes into FT8 TX and read the output power. It should be essentially the same in both cases . If not , something is not right with a 7610 or WSJT audio level setting -
DO THIS ALL WITH AMP OFF until you get same output from the 7610 on FT8 and CW.
I don't anything about the PW1 or 7610 settings etc.
But Is there ALC connection between the 7610 and the PW1 -maybe the 7610 does something different between "data" whatever they call it and CW.