At the moment, Ku'Gath's Fever symptom only grants +150 Favour when spreading a plague to settlements, despite the UI stating this effect applies to ALL armies and settlements. If the favor is to be granted "when a plague is spread", it should behave as the Rot plague with summonable Nurgling units (the resource is granted whenever an army or garrison is infected).
Play Kugath, infect Kugath or any other lord with a plague with the Fever symptom, then transfer a unit from the infected army to another army. Your faction's favour will not increase. If you instead send a plague cultist to a settlement, you will see favour go up.
To fix this, I went into the faction_set_effect_bundles table in the database and changed two cells to the values shown in the images below.
After changing these two cells, Fever granted the 150 favour when spreading the plague to an army. I'm guessing this was an error copy/pasting values when the table was created.