I think its either because of lag, or because I gave the regeneration enemies a bit of invulnerability after they spawn. You could also be talking about the "last breath mechanic" where you have the heal the bastion by doing as much damage as you can in order to get a secondary win condition.
Another unclear mechanic could have been that the corrupted towers exploded and dealt damage to your bastion last second, and started the "last breath mechanic."
Thank you for playing, and if I cleared it up you should definitely try again!