I had a different experience but am still confused. I tried repeatedly to go to Zarah's boat to talk to her when I had both pieces. As soon as she entered, she would go to sleep and if I did talk to her, there was no acknowledgement of my having the maps. We were at 6 hearts worth of friendship. I gave up for a while, put the pieces in my storage and went on with life. Then one day in the forest I talked to her and she got very excited in a cut scene in which I admitted I had found the pieces and off we went to try to find the treasure. It was so random. After a bunch of questions, we ended up digging up this chest. She said the mayor needs to know about this! That was it. Since then, I've spoken to both her and the mayor multiple times with no end resolution. I still have the pieces in my storage. Weird.