The overlap breaks before the mark
The situation
Blue is the outside overlapped boat as they reach the zone, so she owes Green mark-room. Then Blue drops back and the overlap breaks before they get to the mark.
The question
Does Blue still owe Green room once the overlap has broken?
The ruling
Yes. The obligation locks in at the moment the first boat reaches the zone and isn't lost if the overlap later breaks (rule 18.2(a): a boat required to give mark-room must continue to do so even if later an overlap is broken or a new one begins). Blue was overlapped outside at the zone, so she must give Green mark-room all the way round — even though she's now clear astern. Falling behind doesn't release her from giving the room. The animation shows the overlap breaking while Green still gets her full rounding.
Overlapped outside at the zone? You owe mark-room for the whole rounding — even if the overlap later breaks (rule 18.2(a)).
Opens the situation on the boat-length grid — scrub it and see exactly how the boats meet. Free, no account needed.
Rules cited
See the underlying rule: Rule 18.2 — Mark-Room — Overlap Breaks.