I spent 40 turns to dig for treasure and hit a player treasure of 1 gold coin but only 20 turns was used up when even it prompted me to confirm whether I want to use up all my 40 turns to dig for treasure. This happened twice. I'm pretty much sure this is a bug.

01 Days 03 Hours 56 Mins Port 20 80% Failed Poor luck ---> Here I didn't invest any turns, the 20 turns was left over from the previous hunt.
01 Days 03 Hours 58 Mins Port 40 80% Success (Player) 1

This happened to me quite lots.
here one of the ex.
64 Days Baramas 210 84.5% Success (Player) 1
64 Days Baramas 105 80% Success (NPC) 4M

-i dug treasure with 210 turns (84.5 player chances). Unfortunately my crew only found 1 penny of PVP treasure. But, luckily that when this happened i will get 50% of my invested turns. Then I used up the remaining turns(105) for another digging.

I think there some updates made by Jack around this year. not so sure though

Such questions are easily replied if you use changelog search. I do the same. So I found this:
1.8.330 (2017-09-09 13:23:40)

Treasure Hunt minor update:
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

-If the amount lost during a treasure hunt is pretty low, no protection is given.
-If the amount looted during a treasure hunt is pretty low, only half the digging turns are lost.

Thats awesome. Thank you.

That sounds pretty crappy - are there game moderators here?

Well not really. Go to the bottom right of the forum screen and click the team