Sub-issues vanishs when moved with parent to other project

How to reproduce:

1. Setup 2 projects (named it "FROM" and "TO" which have same issue types but different sub-issue types with different workflows.
2. Create FROM-1 (WF: classic.xml) and FROM-2 (WF: classic.xml) as sub-issue
3. Select Move Issue: FROM-1 to project TO.

User is asked to select new issue type, and new status (diff WF). For the sub-issue the user is asked to select the target sub-issue (same is not available any more). User got not asked to select new WF for sub-issue!!! In this case it would be the SprintTask.xml workflow.

4. Then Fire! and BUM! Got the ops message with exception (see in comment)

The movement of the parent was successful (see attachment) but the sub issue is not any more visible in the issue. When I search for all issues in FROM I see the sub-issue - that is not moves - with the parent to the other project. Clicking on it shows a red error box (see attachment). Selecting this issue directly via URL shows: "It seems that you have tried to perform an operation which you are not permitted to perform."

Result: the sub-issue got invisible.