system agnostic – How do we conceptually go about fixing broken gating?

In our campaign, we designed an arctic island with environmental hazards caused by ancient conflict and disaster. Our primary play consists of sitting in the base camp planning logistics and circumventing dangers or risking them to increase expedition progress or gain resources necessary when we eventually cross to currently gated-off areas.

We noticed that we left a loophole in the gating, and we want to fix it without having to backtrack, handwave it, or redoing any sessions. So we thought about introducing an event that seals off the resources that we shouldn’t have been able to access in the first place.

How do we conceptually go about fixing the gating without breaking immersion or making that event (entirely disregarding how that event looks like) feel cheap?