Skyblock; Hostile mobs invisible?

This has happened on numerous occasions and, it's just beginning to bug me now.

So, after a while of not playing, I decided to log onto my Skyblock and started off a new island, like usual. I had built a small tree farm when I started to loose heart points. At this point, I looked around to see a floating bow and a shadow where this 'skeleton' was. I killed it, yet was soon encountered by a zombie - yet again invisible - followed by a creeper. It definitely is not my texture/resource pack. As I have changed them and they still seem to be invisible.
Help?

 

Moved to 'report bugs'.

About your question: I am sorry, but I do not know a solution to this problem.

 

Ah, sorry! It's fine. I'll try re-logging in, it may just be an error on my behalf, due to no other players experiencing this.