Hey guys! Sorry I am late to the party here. As you found out this is something that is inherent to the way the stitch is formed. It honestly has never really bothered me much. If you look at top stitches in most cars they are slanted and many people including my self prefer that look. It also helps to hide slight inconsistencies in stitch straightness. On the other hand it really bothers some people.
I reached out to my contact at Groz-Bekert about this and if he knew if their was any way to solve this. This was his advice:
For slanted stitches, you will have a difficult time stopping them if you are using a R point on a single needle machine. However, you can try this trick that we have incorporated into the 134-35 SAN 12 LR and S cutting point needles for the 2 needle machines.. I think I sent a SAN 12 Brochure to you in the needle case. Just in case, I have attached it to this mail.
https://www.groz-beckert.com/mm/media/e ... SAN_12.pdf
Looking in the SAN 12 Brochure, you will see that the 2 needle machine has 2 hooks going in different directions. So on the right needle. we ground a flat on the shank that repositions the needle when you tighten the set screw. Since the loop is forming at a slight angle with normal needles, the SAN 12 LR moves the scarf at a angle that puts the loop straight out for the hook to pick up. This helps make a straight stitch on both side of a joined seam. Same idea on the SAN 12 S, but it requires a flat on both needles. Left and right.
Keep in mind that the thread is also a major factor in the appearance of the set stitch. Eye size on the needle can wind or unwind thread which causes different set stitches.
I have not tried this yet so I dont know what direction the needle needs to be turned but they went through the trouble to make special needles just for this issue. I'm sure with some experimentation you could find the right position to get this to work. I would just be careful to check and make sure that the hook doesn't hit the needle if you rotated it to far.
If any one does some experimenting with this let us know how it works out!