Document type editor: field properties reset after a few seconds

Steps to reproduce
1) Login the CMS and go to content/Document Types
2) Create a new docuement type, add a String field
3) Go to the caption field and set a new caption 'My Caption'
4) Do not click outside the field and wait a few seconds
Expected:
Caption stays 'My Caption'
Actual results: Caption is reset to 'String'

Raised priority as this hinders automated regression testing with irregular failures