The reason is, that if there’s no remaining work to be done on the operation, it no longer puts any load against any resource, hence no scheduled resource. I.e., the scheduled resource indicates which resource the operation load is booked against so it is only applicable when load exists.

Could also be worth mentioning that there are two ways on how the remaining load for an operation is calculated:

Does this cause any problems for you? I know there was a problem when starting an operation without any remaining load from shop floor workbench since the Resource field had to be selected manually. That was however fixed to 22R1 release.