Calling WFM_UTIL_CALC_SHIFT_PREMIUM from Formula - doesn't work

Summary:

When calling the sub formula WFM_UTIL_CALC_SHIFT_PREMIUM from within a custom Time Calculation rule (array) it exits the loop and moves on to the next record in the array without error.

I tested by hardcoding the input values which worked and replaced them one-by-one with my parameters and it continued working. Then I made an unrelated change to the formula and its stopped working again.

No rlog statements after are logged.

All time values are converted to decimals before being passed.

Content (please ensure you mask any confidential information):

/*********** Check for Night Hours *************'

CALL_FORMULA ('WFM_UTIL_CALC_SHIFT_PREMIUM'