Sorry for bumping this thread. I just want to add observations.
Emergency services, (big) air traffic, and even enemy cars in gang wars are affected by this issue. They all spawn, yet they rarely ever arrive because they disappear. If we pay close attention, they all disappear at the distance in which the LOD model is supposed to become the actual model. The occupants will disappear in less than a second after their vehicles vanish. If the vehicle successfully arrives, it will almost always spawn for the rest of the session (until you leave or load a save file).
I have a save game in which I am at my safehouse in Santa Maria Beach and the Los Santos Riots (the event, not the cheat) are activated. These are the only times in which I see ambulances/fire trucks appear:
-Rarely after setting the Beagle on fire at the Easter Bay Airport.
-Rarely during the Brown Thunder mission in Las Venturas.
-Standing on a house near the El Corona safehouse while pointing my sniper rifle at the airport.
-Running around Los Santos (from Santa Maria Beach) and firing the M4. Paramedics/firefighters can spawn almost immediately, sometimes as soon as the road directly north of the safehouse.
I opened my game, walked near the sand roller to shoot a person with my Desert Eagle, and I ran to the road north of my safehouse. My safehouse was on fire and I pointed my M4 to the west. A fire truck had spawned in that direction and successfully arrived to put out the flames. I didn't even fire my M4.