i don’t think either is correct. the leap second is an extra second, that’s it. the notation for the extra leap second is lacking so we are “forced” to call it 23:59:60 but that doesn’t mean that minute has 61 seconds. it just means after that minute but before the next one, there’s an inserted second.

it’s far more sane to define it that way than to say the duration of a minute (and hence hour, and day, and week) is variable.