TYPO3 Core

TYPO3 v10 is using a Bootstrap panel for the login form, so it isn't affected.
TYPO3 v11 switched to a Bootstrap card, and cards are styled like this:

.card-body {
    *:first-child {
        margin-top: 0;
    }

    *:last-child {
        margin-bottom: 0;
    }
}

So margins are removed in cards for all first and last elements recursively.