I just made a comment on this issue https://issues.apache.org/jira/browse/FLEX-23915 which has a quick fix.
This always happen when a formItem is added to a form and includedInLayout is false. When it's set to true, it will invalidate parent's size, but while running measure()>calculatelabelWidth(), measuredLabelWidth is already calculated and will fail to update properly. -- João Fernandes
