If you find yourself in a situation where a certain field is not being copied, please check the following:
Is the field included in the exclude fields list?
A field on the exclude fields list will not be updated and will be empty/cleared on the cloned issue.
Is the field in context and on the screen?
A field value cannot be updated if it is not on screen.
To add a certain field on the screen check the following article: https://support.atlassian.com/jira-cloud-administration/docs/add-a-custom-field-to-a-screen/