Overview
When you copy the project using the clone project action, only the project and respective issues are copied but not the issue keys or the order of the issue keys from the source project.
FAQ
Issue keys are unique identifiers for the issues.
Can we keep issue numbering when cloning the issues?
A unique issue key is generated for each issue, when the issues are created under the project. When you clone the issue from one project to the other, a new issue key is generated for the cloned issues based on the serial number of the issue key. You also notice that the order of the issues is different in the source and target projects, as shown in the following image: