When I set privacy.firstparty.isolate to true, websites stop saving first-party cookies

When I set privacy.firstparty.isolate to true, part of PrivacyTools’ privacy-related Firefox about:config tweaks, my first-party cookies stopped saving; websites would no longer remember my login. The description given by PrivacyTools is:

A result of the Tor Uplift effort, this preference isolates all browser identifier sources (e.g. cookies) to the first party domain, with the goal of preventing tracking across different domains. (Don’t do this if you are using the Firefox Addon “Cookie AutoDelete” with Firefox v58 or below.)

Is this intentional?

i think its the cookies settings one, did u test it ?

network.cookie.cookieBehavior = 1
Disable cookies

    0 = Accept all cookies by default
    1 = Only accept from the originating site (block third-party cookies)
    2 = Block all cookies by default