So the issue here is the ConvertGridIndexToGridLocationNaive here, which you told me you could not find in your project, so you made your own (above). I sent you the wrong function there, as you need to use the one overridden in BP_GridManager_Hex, which looks like so:
https://i.imgur.com/MHI4NUP.png
but you should have access to this function already, which is very odd. It is one of the newer functions, so are you certain you are using the latest version of ATBTT (created for UE4.25)?