for clarity :) The bug works according to the following principle:
we attack neutral creatures with any hero, and in some cases (which I personally don't know), their number increases by 1 stack. So, if we attack an Azure Drake and the bug triggers, there will be 2 of them in the next battle, but if we attack 49 bears, there will be 56, not 98, so 7 fairies will turn into 8, and so on.
Probably, this is related to the SOD feature of multi-defeat, which ASM tried to fix in WOTG, but in the end, it turned into a bug with other properties :) so most likely, the bug appears when neutral creatures standing on a cell (x, y) should have a global skill (it doesn't matter if they are Azure Drakes or Gnomes).
P.S. what I wrote is based on personal observations and may easily turn out to be nonsense :) so don't throw stones :p