Simplified, the way the server and net code are written, any lag on the line between client and server (both ways) adds up with the 500ms server tick, which makes it entirely possible to be a full second out of sync between what you client shows you and what the server thinks the situation is. That doesn't sound like much until you realize how much movement and action happens in a busy fight in a full second. Try it with you and a friend on a high latency connection both recording your screen, just try to run exactly next to each other. To have the server see you as running right next to each other, you will have to be running well ahead of your friend on your own screen, and your friend will at the same time see themselves as being well ahead of you on theirs.
That also happens to be basically how most of those situations happen where you are screaming because during a spread mechanic, someone is chasing you with their target marker, while that someone is behind their own computer also screaming because on their screen, you are chasing them with your target marker. Since both of you see yourself as the one in front and the other as the one chasing, you will both decide the thing you must do is sprint ahead harder and hope for the other to turn around. Since you both do that, the silly circus continues until the computer applies the mechanic and lets you know you killed each other. Seconds after which you will see the other person catch up to your location as the server's latest updates on everyone's position come in (now that you are both dead and no longer moving) and you will lie happily tanking the floor side by side.
But anyway.
To kind of make this look less jarring, they decoupled snapshotting (this determines who the attack is about to hit), animating the attack happening, and having damage applied (after the server does its calculations), into three separate events so that you don't have to feel like you are fighting in stop-motion cinematography. But the result of this is that depending on length of animation, connection ping, and your timing relative to the server tick, the timing of when damage numbers are shown can drift from the animation associated with it.
Which, I will admit, is not at all what OP posted about but I misread that as that there were supposed to be three damage numbers and only one of them was showing while the animation was happening. On second reading it does look more like they mean that the damage from the three hit animation is processed as a single attack and therefore returns only a single damage number. That last one is an easier problem to solve, but since the monk triple punch is still just a single attack in the code, I guess they could make it pop (1/3 damage) three times?