I was the inside boat and rounded the leeward mark cleanly — how am I disqualified?
Mark-room covers room to sail to the mark, round it, and leave it astern — but the moment the mark is behind you, Rule 18 switches off. Now you're just the windward boat, and Rule 11 makes you keep clear of the leeward boat. If she luffs onto her next leg and gives you room to keep clear (a seamanlike change under Rule 16.1) and your boom still catches her, the foul is yours.
<iframe width="560" height="315" src="https://www.youtube.com/embed/z5JroOENqzU" title="Sailing Kiwi" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
<p><a href="https://sailing.kiwi.nz/leeward-mark/mark-room-ends-at-the-mark/">Source: Sailing Kiwi</a></p>