Please get some more experience in pvp. Then check if it is still the same. Ninja could have cc resistance, they have their own shield so attacking them does 0 dmg for 30s if not broken. They can use sprint and they can use shikuchi to jump far away 2 times. If people are out of combat long enough they will autoheal to 100%, maybe check battle log if there was not another healer somewhere or redmage healing him. Do you autofacetarget targets? If it says invalid target you atleast must have looked at him, was line of sight given, was there a height difference? Was he out of range? Are you using macros? I could go on and ask more and more but, the thing is you haven't thought about all this yet so it is more likely that there wasn't a glitch or hack, but just you not understanding what happened really. These quick conclusions happen to the best people in PvP though, don't worry.
And ok KageTokage show me one of those aimbots please. I doubt you want or can, but even if they exist, they have no value in PvP lol killsniping isn't what makes u win and would be hard even if such a bot exists. I haven't seen bots since stormblood release btw.
And to OP again, I reported that bug even a year ago, wasn't accepted though pepehands
You can see the spinning edge on me and hyoton (but its a range skill so whatever, still ninja skill though). In the enemy party list you see him grayed out and my macros that target him show the error "Target not in line of sight". The ninja infront of me is my teammember. The thing is he has a ninja dot on him from my team, so he is definetly attackable by my team, just cant be seen(therefore its clientsided for me only). So this bug totally exists. Because this guy was/is the best ninja player on chaos back then, he clearly doesn't need to hack