Proxies are indeed inconsistent. If you get someone to 0.5 and they immediately kill themselves or get killed by a baddy, the kill is proxied; either within like two minutes or so. However, if someone waits a good bit and then kills themself, the kill is not proxied. I guess this happens since this system thinks the player has disengaged from direct combat. This is both good and bad, because players can take advantage of this to be petty and not give someone, or anyone, a kill. Or, for genuine reason, don't want to fight anymore or never wanted to fight and have simply, and properly disengaged with no intention to return.
It's hard to decide, because if the proxy time were to be lengthened there may be more accidental or undeserved kills. I don't know man.