Buried treasure

So, my question may sound a bit odd, but maybe someone can clear up my doubts.
One of the joys of online games for me personally is completing achievements. A few days ago, I started leveling my character again after a very long break, during which the game seems to have become more active. While visiting Gateway to Khitai, I came across the quest “What Should Remain Buried,” which is connected to a treasure-hunting achievement. The achievement requires digging up 10 treasures at 10 locations, while the quest itself only requires 8.
I turned in the quest after completing it, and now when I try to interact with the buried treasure spots, the game shows the message “The object is locked.”
My question is: can this achievement only be completed while the quest is active, or is this a bug? Or am I missing something, and is there another way to dig up these treasures? Interestingly, I seem to be the only one who has run into this problem, because I haven’t found a single mention of it or any questions about it online.

You can buy a shovel from another player. Maybe you find one in the trader or ask some other player to buy one for you.

some vendors can sell you a shovel too.
keep a shovel in your bag and you will be able to dig all the hidden treasures except those that you need a specific key to open.

I see. But where do you get this shovel anyway? I tried to find information about it, but found nothing. I bought one from a trader, but I’d like to know for the future where they come from in the first place. Is it random loot from mobs?

In Pin-Pin, there is a cheeky little merchant next to the closed portal in the north of the town that sells shovels and combos, amongst other knick-knacks.