I had that happen as well on Bleakrock. I hit the R key to get it, pop up window goes away, but apparantly so did the fish >.< And that silly thing would have counted towards a fishing achieve.
I found three blue fish, all didn't go into my belly or backpack I was so proud when I got the first and looked for about 10 minutes in my backpack, thinking I did something wrong as it just disappeared ... I thought it did behave like a lore book at first, but nope.. no fish book there either.
So its a bug yes? Good to know, I hope its fixed later ^^
I actually put a ticket in on that a couple of nights ago. My conspiracy theory is that some players unexpectedly find a blue fish in their inventory, shrug and figure they must have caught it and not noticed.