I'm sure it's just lag...but maybe not. I've never seen this in this game before.
Whenever there's a big group attacking me, and I'm spamming stim packs, I'll see my hp at about 1/3 or 1/4 of the max...but my stim packs will stop healing me, and I'll be dead on the ground. One time my health was almost at half when I was dead. It's happened multiple times to me, and only in the new shipyard maps.