When I invested 5 turns in "ask the port locals" of treasure hunt. It gave me the result of 10 turns, i.e , your port maximum treasure increased by 1M, or so, but usually, I saw that it increased by 0.5 M. If it's a bug, pls fix it, or it will give unfair advantages

It gives a plusvalue back depending on the port. If a port gives +1M for 10 turns, lets say, another port might give +2M. It is ok, it depends.

Also, if it gives a +0 then that means you reached the max value in that port (and max depends on the port too).