TLDR - I applied a socket to my neck at the beginning of a battleground. When I left the battleground the socket disappeared.
I’ve read a lot of the bugs about not being able to apply a socket to the PVP neck. This situation is slightly different, as I was able to apply the socket.
SUGGESTION: I was applying an enchant for my boots at the same time within the BG and there was a system message basically saying you can’t apply an enchant in this zone (or something to this effect). I think this same approach should be applied to the neck sockets.