Alright so yesterday I got my new nemesis and was daring enough to give him the Void power set! Luckily on that day, there was a nemesis Alert mission so I went with a friend and lucky enough, my nemesis was there! So the whole group fights him and once he loses one bar of health he teleports and the Alert mission is done. Didn't care, glad to be finished with it!

Today I got a Nemesis clue and it was the one with the mission called Bunker Buster? Well it's that one where PRIMUS gets taken over and you have to clear out all the rooms and rescue five or six superheroes plus the PRIMUS commmander. So my boyfriend and I clear all the room and FINALLY get to fight my nemesis on my own! Two problems happened. All the heroes that were rescued didn't fight in the battle. My nemesis only had two people on his time and last time I did it, there was five including the nemesis. Also he healed sooooooooo damn much! And when we finally got a system to weaken him, once the nemesis was at one bar, he poofed! And the cruel thing is that the mission didn't complete! SO I have to do that all over again.... anyone else having issues with their nemesis?

That's not the final showdown with your (current) Nemesis. You'll know when finish that mission.

Until you get that mission, your Nemesis will continually escape before being defeated.

Except that isn't what is happening. I tried fighting my newly created Nemesis (they have Darkness powers) and during the first encounter once the first bar of their health is nearly gone they'll begin to cast a power and then just disappear, like they've just teleported. This isn't the same as beating on them until the cutscene plays in which they leave.

The same thing is occuring with Dual Blades nemeses.

I am having the same issue, also with a Darkness nemesis, but in the Deathray Demolition mission. I haven't particularly noticed mine casting anything before disappearing, but I'm not certain he didn't, either.

I also noticed, on my most recent attempt, that his max health jumped to 200k partway through the fight, like he has in an Alert, for a while before disappearing. I'm not sure if that's related or a separate issue, though.

I wonder if the Nemesis Alerts are causing some issue with normal Nemesis missions then?

It could be related to the Alerts, yeah. But it's also happening in Alerts apparently, so it's not just that Alerts broke the normal missions. I'm not sure if it's isolated to Darkness nemeses, either.

It was happening with my Darkness nemesis in Alert missions. It stopped happening after I switched him to Sorcery.

There's 3 or 4 Nemesis power sets that are completely bugged right now. They'll outright vanish or their AI scripts will break down if you use them. There's no other solution short of changing their power sets.

Long-standing issue (month or two), sadly.