Thank you for posting the program !
The system has three PowerFlex 755's configured as CIP Motion axes (Shuttle, Tension, and CAT) and four PowerFlex 755's configured as ordinary variable speed drives (LETOFF_1, LETOFF_2, TAKEUP_1, and TAKEUP_2).
The drive that's not functioning as expected is Letoff_1.
All of the Tasks in the program are Periodic; there is no Continuous Task in the program.
The absence of a Continuous Task tingles my Spidey-sense; it's uncommon and there was a communications-related bug that mostly affected diagnostic I/O modules in some applications that have no Continuous Task. I would generally run the Main Task as Continuous instead of as a 2 millisecond Periodic Task.
The Reel Up features are executed in a 50 ms Periodic task, and the Letoff and Takeup drives all have 20 millisecond RPI values.
Calculated_Letoff_RPM is used by the Letoff_1 Program in the 50 ms periodic Reel_Ups Task, but it's actually calculated in the Math Program, which is executed after the Letoff_1 Program. So at least those calculations can't interrupt one another.
I was a little surprised to see the Letoff_1 drive configured with as many Output datalinks as it is. In particular, the Speed Reference A Setpoint is configured as an Output Datalink (Letoff_1:O.SpdRefAStpt).
As far as I can tell, that value is 0.0 and it's not linked to any logic. It should not matter because P545 is set to get the drive's speed reference from the network, not from a fixed setpoint.
So.. I'm not stumped, but I'm further along with my head-scratching.