I had a Fallen Spire game. I definitely have Core Shield Generators disabled; I looked at "Galaxy Stats" and it's disabled, and I've also explored all the planets and there are no core shield gens. However, as soon as I send my fleet to the AI Homeworld, the Guard posts immediately have "External Invulnerability". If I just send in a normal fleet then it doesn't happen; the guard posts are not invulnerable. Is there a known bug about this?
