After updating to 2015 R6, text manually entered in an Item custom field is removed/cleared after tabbing, using the arrow keys, or clicking in another field. When Custom Field Labels have been deselected in the Set up Custom Fields for Items window, all subsequent fields are essentially inactivated. A Label field with pre-filled text will remain unaffected.
Temporary fix - Select all Custom Fields for Items for Use, then modify or customize your template to hide the unused fields.
In Pro/Premier - Edit any Item, click on Custom Fields then Define Fields to launch the selection window then select all.
For Enterprise Users - Edit any Item, click on Custom Fields then Define Fields to launch the selection window then select all.
Thanks to Intuit for the information contained in this article. Refer to Intuit Technical Support Knowledged Based article ID SLN94259 for more information, or to subscript to get an email when updates are available for this bug.