So I was running along near whiterun (near the Underforge's hidden exit) when the encounter zone spawns a fight between the Vigilants and a Vampire. Strangely the vampire immediately yields and flees, not attacking the Vigilants which means that whenever I encounter them the latter eventually killed it. This happens to me all the time until I get myself involved, upon which the vampire becomes hostile and wipes out the Vigilants in seconds. Is that normal behavior or a bug?

The odder thing is when I did get involved, one of the Vigilants dies and sinks beneath the ground near some rocks. I happened to step on the same area when I suddenly sink as well, fell for a few seconds, then suddenly I'm on a hill southeast of Fort Greenwall in the Rift! My radar also still shows the vampire's general direction, but as a real small red dot, so I assume the Vamp is still there, which also meant I cannot fast travel until I enter Sneak Mode and become undetected. I'm a bit scared to replicate it but Is this also a documented bug?