Multiple pdf's not open in same window - Reader DC

Hello guys, i have a problem with my Acrobat Reader DC (free version). On Windows 8.1, despide the "Open new documents in the same window" option is checked (Preferences - General), however, when i'm opening multiple pdf's, there are as many windows as the numbers of pdf's opened in the taskbar (big buttons - always combine), as if there are multiple instances of Adobe Reader, despide the documents are actually shown in tabs inside it (see the difference in the printscreen between Adobe Reader and Notepad++ which also has multiple documents opened inside it). How can i solve it? It's a bug? Thanks!