I since discovered the cause: On individual registration, the id/name of the input is correctly identified to the script function, but on group registration, the id/name of the input is affixed with an underscore and sequence number (e.g., _1) that is not recognized by the script.
I use "onblur" to trigger the function, so I need to discover a way to get the function to recognize the specific input element(s) when triggered. For instance, (1) identify the input that triggered the blur, (e.g., YOB_1), (2) identify the corresponding input, (e.g., Category_1), (3) and run the test with those two inputs. Any suggestions dearly welcomed.
I'll post the solution to the forum when I discover it.