When changing Position an additional Grade is getting added due to which it is effecting the payroll

Summary:

After the shift diff update in November, the following week we had a few positions not receive shift diff.  When researched, it was discovered they had the new grade and old grade. we removed the old grade and we were able to process without issue. Thus the assumption the second grade was causing the issue.

With the issue being the shift diff not being received during the payroll calc processing, can someone please suggest how this is being tied to positions with multiple valid grades. 

The shift diff calc does not use the position valid grade to determine shift diff rates, but what is the reason we are having this problem? And why removing the valid grade from the position has any relationship to solving this?