Bug happens if u walk far away enough from the bosses body. If I come close there is no visual bug - go far away enough its there. Its the pestilence shallows boss (first one that spawned for me)
Well I have 20% pestilence spread left in this area but no enemy in sight. That would explain a lot ^^
Have u checked under the shallows? 
in the sewerage? I walked over the whole beach. If theres a hidden area I didnt discover it yet. Spoiler me if you want 
lol just saw the trapdoor now … wow
Yup, they added that in this patch. The trapdoor was always there but no accesible before
I only saw the trapdoor near finleys prison and the one in the woods above the first darak fight area. THX 