I buried a stash and there is no visible dirt mound. Even though there is nothing special I did that I know of. It is just buried in the middle of a little grass area.
Upon relogging it was still there. And after server restart it was still there.
Further inspection during night time and using a red head torch. I have found the stash dirt mound to be upside down. When viewed from the top, the bottom makes it invisible. I laid prone and could see underneath of it, seeing the dirt mound.
During the day time it is the same.
All pictures provided below should help explain and show the mysterious dirt mound in action.

https://imgur.com/a/PCHGSwV

https://imgur.com/a/Sq5Ap7j

https://imgur.com/a/mgZtqHk