Sorry to open this as a new issue (I cannot re-open the closed issue).
This still (newest version of Preferences Monitor) undoes the changes to the user agent on startup even if you add the string you kindly suggested. And for some reason as it does it on startup there is no option to whitelist. However, it is no longer an empty string after the change (it returns to being the actual user agent).
Sorry to open this as a new issue (I cannot re-open the closed issue).
This still (newest version of Preferences Monitor) undoes the changes to the user agent on startup even if you add the string you kindly suggested. And for some reason as it does it on startup there is no option to whitelist. However, it is no longer an empty string after the change (it returns to being the actual user agent).