Phoenix was never checked by a vet upon her return, and never given horseshoes before being saddled. She could have had cracked hooves, bad teeth, ticks, worms, knotted mane, contagious equine diseases. They never even quarantined her.
Jack's horse Sirocco meant the world to Claire and sired Blaze who foaled Phoenix. Blaze is completely AWOL, and Phoenix returns from running with wild brumbies looking like a champion showpony. Not only is that impossible, it insults the viewers' intelligence. Tess would have searched the earth for those two horses of that bloodline.