section of the spec that defines what happens to block margins that are between block level boxes:

When an unforced break occurs between block-level boxes, any margins adjoining the break truncate to the remaining fragmentainer extent before the break, and are truncated to zero after the break. When a forced break occurs there, adjoining margins before the break are truncated, but margins after the break are preserved. Cloned margins are always truncated to zero on block-level margins.

But what about margins that aren't between block level boxes, such as a margin-block-start of the first child of a block formatting context (BFC), or a margin-block-end of the last child of a BFC?

Also see css-break-3Current WorkCurrent Work