When I take a screenshot in ChatGPT, it shows a "share this chat?" toast at the top.

The toast shows up after the screenshot, but my phones's long screenshot feature captures the top part a second time, so the top part of the chat becomes unreadable.

Yup, that's by design. They want to force you to "share" so they can both track who you interact with, and also try to convert the other person into a paying customer.