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BFG tracers use the living player location rather than the original corpse that fired the shot. When the bfg ball detonates, tracers fire from the living player even if they were killed after shooting the bfg and have respawned somewhere else. The bfg tracers should use the corpse of the player that shot the ball whether living or dead.

This has evidently been a zdoom bug for sometime. I have mostly played zdaemon and odamex over the years and just now started zandronum. I noticed this bug right away on zandronum and posted on their bug tracker:

https://zandronum.com/tracker/view.php?id=2889

After some investigating, this happens to be a bug from zdoom that zandronum inherited. It seems insignificant for single player, but in multiplayer it matters a lot, because you can correctly avoid your opponent's bfg and kill them only to have them respawn and have their bfg incorrectly target you. I tested and made sure this is still a bug in version 2.8.1 of zdoom. I hope this can be fixed! Thanks.

Not easily fixable. This is caused by a blanket pointer substitution that relinks everything that pointed to the old player to the new one. I have no idea what needs this and weeding out this one particular pointer isn't easy, because it's just the target member of the projectile.