How to reproduce:

Create in the first jira project a new issue with sub-tasks (sub-issues). Create a second project that as different issue types and sub-tasks than the first project. It is important that the issue types and workflows of the first project are NOT available in the second project.

Then move the issue with sub-task to the second. You will get asked to select a new status for the issue in the second project (but not for the sub-tasks). Also fields are shown that are new in the second. Message before pressing 'MOVE':

Note: Sub-Tasks associated with this issue will lose data stored in fields not applicable in the target.

Now everything including sub-tasks are moved but they have an issue type and workflow that is not available in the second project. To get rid of this is, move sub-task to issue and back. Then the right workflows are selected.

Scanned the log file but there are no Errors / Warnings during that movement. Tried it again on test projects, then the dialog for mapping sub-tasks was shown (see Snap02). But the workflow was not selectable (see Snap01). All were default mapped to standard 'jira' workflow.