Not sure. I haven't come across 'Joy Story' being identified as a Pixar short. Maybe it's a relatively new or less popular one that I'm not familiar with.
It's possible that the team at Pixar felt that the full-length Toy Story 4 told the story completely and didn't require a short to enhance it. Or perhaps they wanted to save their creative energy for future installments or different projects altogether.
It could be that the story of Toy Story 4 didn't lend itself well to a separate short. Or perhaps Pixar had other projects in the pipeline that took priority.
For short story collections, you can refer to the following suggestions:
"Battle Through the Heavens": A very popular fantasy novel that narrates the growth of the protagonist Xiao Yan. After experiencing a series of challenges and difficulties, he finally became the powerful ruler of the Dou Qi continent.
2.'The Peak of Martial Refinement': A fantasy novel with a martial arts theme. The main character, Xiao Yan, continuously breaks through himself to become a powerful martial artist in the process of practicing martial arts.
[3. The Devil's Sky: A novel with a fantasy theme. It tells the story of the main character, Xiao Yan, who, after crossing over to another world, fights with all kinds of monsters and enemies and finally becomes a powerful martial artist.]
4."Covering the Sky": A novel with fantasy and science fiction as its theme. It tells the story of a mysterious world where all kinds of powers interweave together. The protagonist, Xiao Yan, constantly breaks through himself during his training and becomes a powerful martial artist.
[5. Douluo Continent: A very popular fantasy novel. It tells the story of the protagonist, Tang Wulin, who continuously broke through to become a powerful soul master in the process of cultivating his soul power.