I sometimes have that glitch too, I was able to fix it if I zoom into some place in the middle of the map and then add it. Most times it adds it where the cursor is. If not, I delete it and try again later. If it still happens then this is a way you can fix it: if it goes into the corner, you can use the XYZ coordinates to move it that way. You can also type in your own coordinates and to do that, move your cursor to around where you want it to go. At the bottom right it shows you the coordinates of your cursor position with it being X Y Z Yaw (not important). Copy the first three numbers (I suggest on a piece of paper) and then plug those into the X Y and Z slots and then move it with the arrows to where you want it.
