The fact that the start() method failed was the clue because ...
I had recently introduced into the batchable start's QueryLocator the field OrderItem.Product2Id
The batchable class was written as of V38.0
The field OrderItem.Product2Id needs the Database.getQueryLocator() method to be executing with at least V40.0 to not cause the error.
Updating the batchable class to V56.0 solved the issue
We could generalize that referencing a standard field not introduced until Vxx can not be used in a batchable's getQueryLocator method if batchable is running at Vxx-n, n >= 1 but I did not test this generalization.