That's pretty odd that one achievement is considered complete while the other isn't, even though they share a common requirement. O_o
I haven't heard of this bug yet, but many of the pet collecting achievements haven't been working properly for different people. It could be a random occurrence. I would put a ticket in to report this bug and see if a Game Master can provide more assistance/info.
Feel free to browse through Perks N Peeves
, my vanity pet collecting blog!