5-S doesn't stay unlocked

[Marfolog]

Found the entrance to 5-S, checked the mission for a bit and then left to fully complete it later. After checking missions again the secret mission was still locked. I tried it again, I could play the mission, but it remained locked afterwards. Is it a bug or am I just missing something obvious?

[Brickybattler]

just complete the mission then u can play it again whenever