“radius of treasures” probably refers to the area around the coffer. As you get close to it while holding a treasure and try to place it, a large golden ring will show up around the coffer. The treasure just needs to be placed somewhere inside that ring.

Also, some possible reasons why a treasure might inexplicably disappear:

Someone used the exchange function on the coffer to turn it into gold coins.

The purge got close to your coffer and converted it into gold so they could carry it away.