Hi,
Looks like core 8.4 and jquery 3 cause more problems. I've noticed that next-prev buttons always have opacity:0 - http://prntscr.com/hg9a5r
The second problem is the wrong slider structure: http://prntscr.com/hg98jl looks like this doesn't cause a problem on our main website, but I didn't tested all our sites.
Thanks.
Are you using the latest version of MD Slider?
If yes, please send us your site url, admin account to check