Line 51 is my current issue.

const {
    reasonCodes,
} = useSelector((state) => ({
    reasonCodes: state.reasonCodes.data,
}));

reasonCodes is always already populated with the full list of ReasonCode values when we first get here so we never enter the !reasonCodes.length condition to fetch just the ADJUST_INVENTORY ones.

I'm assuming I need to switch this to something like: reasonCodes: state.cycleCountReasonCodes.data,
but I don't know enough about React to know how to set the state properly. Something with "useState" it seems. I'll keep poking around but if anyone has inslight, let me know.

I am not a big fan of mutating values, so I prefer something like:

const url = `/api/reasonCodes${activityCode ? `?activityCode=${activityCode}` : ''}`

I think you should be able to do something like this:

        const reasonCodes = res?.data?.data?.map?.((reasonCode) => (
          {
            id: reasonCode.id,
            value: reasonCode.id,
            label: reasonCode.name,
          }
        ));
        dispatch({
          type: dispatchType,
          payload: reasonCodes || [],
        });

When something from the call chain is null/undefined, it will fall back to [].

Note:
|| use the fallback when the checked value is falsy,
?? use the fallback when the checked value is null.
In this case, I think reasonCodes will fall back to null.

Won't that change break the use cases for reason codes in other places?

This is just an idea, not a change request.
Maybe it will be worth having. a place where we can store our validationrs to keep the classes cleaner? I can see that this closure is duplicated.