I've fixed a number of bugs with the Cabal since the 1.5.0 update. Lots of bugs, in fact. It's at the point that I don't think there are more bugs to find, and then you come alone with a bug that is literally inexplicable.
There is a special flag given to fleets that witness, either firsthand or given the information secondhand by a same-faction fleet you've attacked, a hostile action you took. The flag interacts with the Transponder ability to make any fleet with that flag nearby trigger a reputation drop to Hostile if you turn your Transponder on. There is, however, a separate flag that suppresses this behavior.
The Cabal have, for several versions, had that latter flag whilst masquerading as other factions. Therefore, they will, say, become spooked if you attack a Cabal fleet at some point and the grapevine reaches them, but a fleet that's masquerading as another faction will never care about hostility events—not to nearby fleets or even themselves. This leaves no opportunity to flip hostile unless, for whatever reason, you've genuinely attacked a Cabal-presenting fleet at some point.