How to "Investigate rock arches on the Mun" in Kerbal Space Program?

I'm doing historical missions, the "Investigate rock arches on the Mun" mission.

I've reached the Arch, taken samples, EVA, Crew reported, landed really near it (<100m), climbed it's top + crew report + Samples.. and still the contract is not satisfied.

What am I missing?

UPDATE

I think it's a bug. I've been there a second time, still nothing happens.

Screenshot added: enter image description here


Have you visited all the arches on the Mun? The easter egg KSP wiki page says:

Three giant rock arches can be found at 12° 27′ 2″ N 39° 11′ 45″ E, 12° 28′ 18″ S 140° 57′ 40″ W, and 2° 27′ 47″ N 81° 31′ 51″ E.
You have only explored one arch. If you find the others and explore them, you should complete the contract.