Looking into this! It is possible something got overlooked. Thanks for reporting this.

(In the meantime, please do not fill up your pet slots while you have a pet in limbo; glitch abuse is a warnable offense. Not that you need the reminder, but I gotta put it here just in case)

Looked into this! It looks it does prevent you from creating a pet if you have a pet in rift or steamwork limbo, but adopting an existing pet from Adopt R Us was overlooked. This has been fixed.

The pet slots count on your pets page has also been updated to show the accurate number (in both cases)