[2019] Why do enemies become The Flash when they are low?

This happened several times already and it's really frustrating.

You start shooting someone, they might run to cover or just rush you, you kill them, all good. But sometimes, specially in harder difficulties where the enemies' HP pools are higher, whenever they hit ~10% HP they just start running around like crazy way faster than they normally run. Its like the game wants them to find cover asap but they just don't work like regular enemies anymore.

Maybe it is because of the speed, but you can't even hit them properly, it feels like half the bullets just pass through them.

Is this a bug or it is intended?