Regrettably, field “Tag Selector” of type doesn't support auto completing. Please use following procedure instead:

Please create new field for Tags using 'New system attribute' button and then choosing 'Document attributes' as 'Group' and 'DocmentTags' as 'Attribute name'. You can inspire by 'DocumentTags' field of CMS.BlogPost document type.

This issue may happens if you specify tags into field of 'tag selector' type and if you have enabled Cache content setting in 'Settings'. This issue is caused by old tags list is cached in tag cloud. You may save blog document again or restart application in Administration -> System section. You may also consider setting Cache content to zero minutes during development phase.