I have no clue if I could reproduce this. It just happened so suddenly and I'm not sure when it started, but I was around 4ish (real-life) days into my IC2 1.15 game when animals simply refused to spawn in/near the chunk where I keep all my machines. Batboxes, copper and Glass cables, MFE, generators, all the usual stuff you expect. The problem is I also have Portal Gun, BC 2.2.0, and a few other minor mods installed. I'm gonna try again and replicate this error with a bare minimum IC2 1.15, but whatever happened, I think the chunk(s) that held all my machines somehow corrupted?
Edit: I have started a new game with the same set of mods. Animals spawn normally on a fresh map. They also spawn normally on the same (corrupted) map if I travel far away from my workshop. Hostile mobs spawn normally everywhere though.