This problem occurs due to overflow. The game allocates 2 bytes for the stack size, so the maximum number of creatures in a single stack should not exceed 32767 (if the most significant bit is a sign) or 65535. When more creatures are added, an overflow occurs, and it wraps around to 0, causing the numbers to become negative. I (and others) explained this in detail in a certain thread. The same can happen with experience and resources if you play on the same map for a very long time and over-level a hero or accumulate too many resources.
By the way, what map is it? And in which version of the game do you have 80,000 liches? Most likely, it's WoG: it has additional features to quickly gather armies.
Regarding the topic, the left hero still has a considerable number of liches; why doesn't overflow occur there? Initially, the right hero had an army of over 300,000 liches in a single cell.
By the way, the map is quite interesting; such a large number of liches isn't just for show; I recommend it.
