The player object is passed by reference as Target (as the positional interface type) to MoveToPositional() and then its transform is updated every half second in the loop.
As I say, ideally there would be a MoveTo() equivalent that updated its target without starting and stopping a separate MoveTo() command (which is also slightly glitchy). 
I have seen a number of people post a variation of this observation. I wrote Sleep() and it is supposed to wait precisely one update frame when 0.0. If this is not the case then there is likely something wrong. 
It depends on an internal Unreal event system and the Blueprint Virtual Machine. It also used to be several different functions that waited - instantly, per frame update, per specified seconds and infinitely - and now all rolled into Sleep(). So any of those aspects or something entirely different could be doing something unexpected. I’ll dig further into that and see if anything is amiss… It might be possible to simplify or make more efficient (and fix if there is an issue) by making Sleep() more Verse specific.