I posted it in "general" but I guess I should repost it here.

I found if a tile gets ransacked, it can no longer be exploited even if you perform the "repair ruin" task in your city. If I real want to build districts outside my city and I do not want to spend twice the production building a hamlet, I will build a garrison, then build districts next to it. The garrison will later be ransacked so that I can reuse the tile where garrison was first built.

However, it seems as long as a tile ever get ransacked, it can never be exploited again even after repairing.

Photo 1: a tile should be exploited but in fact having no yield.

Photo 2: a successfully exploited adjacent tile, for comparison