Although i have ~20 windows of one application (Firefox) opened, the taskbar window list shows just 16 (in order of usage?). I've encountered this on Kubuntu 16.10, 17.04 and Linux Mint 18.1 as well. Kubuntu 16.04. worked fine. Tested with fresh installs in VirtualBox.
*** This bug has been marked as a duplicate of bug 379037 ***
fresh install of 16.10 seems not to be affected. behauviour seems to start with the design change as documented in the attachement of 379037. thx