Part of my standard security practice:
    Stronger tech prevention (due to the isolation) of cross-site tracking, or possible exploitation.
    Also better session management.
    .
    Nothing specific to Tuta.
    And people do have other causes too.

    However, the state of this setting is much inconsistent with:
    Generally most other settings, that do sync to remote.

    And from user experience aspect:
    It's better to default this new setting false, for less factor of astonishment.
    (the added new behavior may surprise user; aforementioned)