That sounds like a bug. I’m not able to replicate it, though. Could you give me the steps to reproduce the bug?
You should be able to do all of this by modifying an ability. If you take BP_Ability_Move as a base, you could store the current displayed path when you press a key (storing the current PathIndexes array). You would also store the grid index of the marked tile and the movement cost of moving to this tile. From then on, in the hover event, you would run pathfinding from the stored grid index, with a move range equaling the move range of the unit minus the stored cost. You would then append the stored path indexes of the indexes generated in the hover event when displaying the movement spline. When choosing to move you would get the entire current path and feed this into a the MoveCustomPath event of BP_Unit. This is the general approach I would use. I know it is a bit to follow, but try it out, and if you run into difficulties let me know and I will try my hand at making it.