Enabling freeMode the last slide is showed only at certain viewport width.
Here the demo
https://codepen.io/gooogooo/pen/LYbYRYL?editors=1010
To replicate: bring the slider at the end and try resizing browser window width. You'll see that at certain width it shows the last slide, at other ones not.

Update to 6.4.10
Oh man thanks! I wasted like 3 hours on this! 馃槴
Most helpful comment
Update to 6.4.10