The reason for this issue is that in one location (cell 2,76,0 and nearby), there are Orcs on Boars, set to be friendly to anyone who approaches them. There are 9 stacks of 4000 each. A total of 36000. And in my case, Purple took them and soon placed them all in one stack in the city of Vivian.
The game incorrectly performs the creature transfer operation if, as a result, there will be more than 32767 of them in the new location, which leads to a numerical overflow and a negative quantity.
So, after passing through the nearest monolith, I took this city by storm, and I received a negative amount of experience for killing a negative number of creatures. :) I haven't seen anything like this in regular maps yet. Fortunately, the experience reduction is relatively small, and I compensate for it by visiting the tree of knowledge, but pay attention to such moments if they occur for you.
For reference: Orcs on Boars are not required in any of the quests, so you don't need to worry about them (I checked in the Prospector).
(Василь Симоненко. Люди – прекрасні)