Why does an end screen show in YouTube Studio but not on my Short?

That can be normal. A classic end screen may be saved in Studio, but the Shorts player has its own layout and YouTube says end screens can appear differently depending on the viewer's device and context. For a Short, use the Related video setting instead: it puts a clickable link below your channel handle in the Shorts player. The linked video has to be public or unlisted, and your channel needs advanced feature access. Keep the end screen if people also watch the video on a regular watch page, but don't rely on it as the Short's main call to action. Also check that the video isn't marked Made for Kids, since end screens aren't shown there.