remaining monster in the old fields

Hello and thanks in advance for some help. Im just a noob and courious about it:

In the map "The old fields", even after a long, long search, monsters are ALWAYS remaining. Why is that? I've found all the stone monsters and all the others... It only appears in this map.

Is it because of an open task or so? There are always between 3 and 5 monsters left to hunt down.

...I know there is no need to hunt all down! ...just courious.

https://ibb.co/Db2t4Qv

bug 1.802.889.383