Funny thing is, I've killed people when they were spawning, and those i-frames were irrelevant because they started shooting which took their invulnerability away. But to avoid getting shot in the back is to find the closest alley that you can find. The spawning system isn't nearly as terrible as GTAIV was so getting away when it gets hairy is much easier. But in those God-forsaken no-cover areas is where I feel a little exposed and just try and run as far as I can, if I start to hear sniper shots whiz by then I zig zag and pray lol.
You and I must have different games, I'm pretty sure I didn't miss because I see the bullets hitting their character but they can damage me. Happens in deathmatches too, maybe its lag or something
Same happened to me a few times. Whoever I just killed respawns right next to me so I start shooting and aiming for the head straight away (even do a barrell roll for safe measure) but I can't damage them while they were/casually walking and shooting at me (and damaging me).
I always assumed the same that once they engage in combat their immunity stops, but on a few occasions it didn't feel like it happened way. Maybe it's not just shooting but it's once the first bullet hits and damages me that the invincibility buff ends. In which case, whomever re spawned last always gets the first hit while in that state. Which is fair I guess. Could be lag too. I experience lag here and there so I wouldn't put it aside.
Just would be cool to know either way