When “An unexpected emergency” mission appears it is said that there are an outbreak.
Corruption Mechanic in Geoscape should start at that moment, forcing player to do the mission asap to unblock Mutoids and treatment for corruption. As is it now, player can delay months before doing it, building stronger teams and Myst Reppelers in bases turning the DLC much easier.
Corrupted Horizons mechanics is broken! | Voters | Phoenix Point