You are talking about ChatGPT, the web chatbot at chat.openai.com?
If you see other conversations as a result of others using your account, they’ll remain until you delete them.
You also don’t have a method of ending session on other users, you have to wait for them to time out. Significant security concern for the already security-impaired.
This is maybe the third or fourth in a day with mystery chat history appearing. It could again be a database problem of other account messages appearing for your user interface.