Jira issues get their number in the order they are created. The first one get PROJ-1, the second one get PROJ-2, and so on.
When the import is done in Tuleap, the issues get a new number that depends on the order they are created in Tuleap.
In order to keep as much consistency as possible, the order of creation in Tuleap should be the same as the order of creation in Jira.
This way, the issues that were created more recently will get the higher number in Tuleap and artifacts created after that will have a consistent number