Steps to Reproduce:

• Create a Field Configuration named Bug and add Affects Version as a required field.
• Create a Field Configuration Scheme named Bug Scheme and associate issue type to bug
• Add the Bug field configuration to the Field Scheme for an example project
• You should now have a project with Affects Version set as required for issue type bug, and this field is not required for all other fields.
• Create an issue with a type other than bug (eg. Story)
• Add a subtask to this issue and attempt to move it to issue type Bug. You will notice the Affects Version field is required, however there is nothing populated in the drop down.

Expected Result:

• The dropdown should allow you to set an Affects Version for the task and the subtasks.

Workaround
If you set the required fields to the desired value prior to the move, the operation will complete successfully.

I've tested and reproduced this behavior against JIRA 6.3.8 and JIRA 6.3.13