this is not a bug, but rather a balancing issue:
In two different maps, when fighting two different factions, I've encountered the same "death wave" of goblins.
A few weeks in (e.g. 6) into the campaign, an enemy hero will have hundreds of goblins, and then they'll either have the ability that boosts damage when an ally dies around them (my penultimate game) or they will have the ability that makes clones of your troops (my very last game).
This results in unbeatable odds where the goblins hit like trucks (the former scenario), or that you will die through an unceasing wave of clones, even if the real goblins themselves died.
The AI seems to privilege this strategy. My latest game was against an Order faction, and they still had hundreds of goblins. I also wonder how they can have so many of them a few weeks in (I assume the AI gets to recruit more troops than the player through inherent bonuses?).
The only real counter I've found is ultimate spells that deal tons of damage on the battlefield, preferably to all enemy and allies (aka all characters on the map), but these spells are rare, even at higher level, and depend on how a hero was skilled/what faction they are.
For what it's worth, I am currently playing the latest patch: 1.5.4. This did not seem to be that much of an issue (although it could just be sheer luck) in my prior games on the prior versions.