Under design customizing there's a place to check or uncheck flexible backstitch. Notice that the words don't appear in edit like Standard so I went looking for it.
Question - you say it will only start on the grid marks - is that ANY corner of the stitch or only one corner? Just out of curiosity, I just did a check and the tips say that if you have it turned off, it will only "hook" to the upper left corner of the stitch and not any other the others.
If you have it where it will hook to any corner, then maybe you're "aim" is off. I've done that many of time but usually when I want the corner and it goes halfway down.
Note to self: When you turn on a switch (my flexible backstitch was actually not checked when I went lookin for it) you must reboot to have the dang thing work.
Now my backstitch starts where ever I start it, not just at the corners. Normally I have flexible off cause it's annoying but this is the first chart I'm designing in Pro and I actually wanted it.