You have to move your camera to a spot were your soldiers are fighting and stay there for a few seconds. If you don't keep looking at the very same spot all the time the blood vanishes after a while. Beheading is super rare and unless you keep searching for it you will never see it.
Hope this helps, but don't get too excited because if blood drops on the ground it is the same graphic over and over again.
I'm playing on extreme preset, but it also works on ultra and very high (I haven't gone lower yet)