Multi-window capable apps missing in the multi-window panel.

Everything works fine before. Then suddenly, I'm starting to feel that's something is not right anymore. The Facebook App is missing in the muti-window panel, even when I edit what should be on my panel, Facebook is just not on the list. But obviously, Facebook App is sure capable of multi-window. Thanks a lot for your help.

Model: Galaxy Tab 4 7.0 LTE