I can see, that the 0/45/90 helpers only are visible and to use if the balloon isn't locked to the alignement shape and You click the leader line point instead of the balloon center.
But what I have found is shown method:
Pull away the balloon from the alignment shape, move it using the leader line end point (helpers are shown) and move it then to horizontal AND alignment shape.
Surely this only is a minor work around to get what You want, and it should be improved by developement.
But on the other side, it works and it does what You need.
PS.: as I have learned and as it is described at the standards, leader lines for balloons must NOT be under 0, 45, or 90 degrees, do not mix them up with geometry lines, hatching lines etc.
I use the toggle grid command to assist in lining up ballons and text.
I previously drew a line as suggested above, but the toggle grid is much faster.
I added a command to the Quick Access toolbar and it will toggle grid on/off with one click.
It is also in the Sketching - Draw command group in draft enviroment.
the only issue here is, that using the grid will not guaranty that the balloons will stay aligned.
Here the alignment element for me seems to do a better job.