We've spent a good deal of time looking into this, as have many other app developers and all the clues are pointing at a bug in the Touchwiz launcher used on Samsung's 4.4.2 ROMs.
Here are some link of others reporting similar problems:
http://levelupstudio.com/en/blog-en/widgets-disappearing-with-samsung-4-4-2-roms
http://www.3c71.com/android/?q=node/2594
http://cloud.tv/apps/hd-widgets/faq/ (search for disappear)
http://androidcommunity.com/kitkat-bug-may-be-affecting-widgets-on-your-handset-20140508/
http://stackoverflow.com/questions/23144968/widgets-gone-after-app-update
https://koxx3.wordpress.com/2014/07/01/widgets-disappearing-with-samsung-4-4-2-roms/
Possible workaround:
http://www.reddit.com/r/Android/comments/1057b1/are_your_paid_widgets_and_keyboards_disappearing