He is suffering from a lack of positional awareness, sorry to be banging on about this for a couple of years but in his first season his natural ability made up for the weaker side of the game as he learning to play at the highest level, having been mostly a CB, throughout his youth career. Moving him to DM and did very well as positionally he just need to adapt to the CB's and had them covering him. Going back to CB and he looks lost again. as he doesnt know where he should have been. You saw the exact same thing happen to Jones(people on this forum actually said at the start of the season that this was a silly comparison as Jones isn't that good). Jones and Dier imo needed to continue to learn at CB if they want to continue as CB. Dier at DM was a great success but he needs to learn a position instead of keeping chopping and changing like Jones.