I had this
exact same problem. Twice. Both times at the end of the game, and I'm talking after spraying all tags, and completing all missions etc. Both times I was forced to start again.
I don't know what causes it, or what fixes it, I just restarted. The second time I completely erased GTA: SA from my memory card in case the bug can spread as you're loading up each save. So far on my third time through I've not had the problem, but I don't remain hopeful. I suspect using a cheat at any point causes it, I tried to use the change model cheat (Thinking it would turn me into a pedestrian or something) and maybe that's what caused it the first time, I dunno.
Good luck with it anyway.