Have you tried this? (works in Filemaker to remove the "antsy borders".)
Open layout mode and clear the "show field frames when record is active" option.
Doesn't resolve your bug but may avoid the need for the script that revealed it in this case.
Thank you for your post.
This problem has been previously reported at:
In essence, a Checkbox/Radio Button field split over repetitions will not allow the second instance of the field values to be selected when the second instance doesn't begin with the first repetition number. In your case, 1-3 and 1-18 would work, but 2-18 would not work.
Our Development and Testing departments are aware of this issue.
I have attached your post to the original report. When any additional information becomes available, I will let you know.