With parenthesis in the number, trying to use autofill will not work at all anywhere. When taking the parenthesis out it works if nothing is entered or if you put your cursor in the first three digits before the first dash (123-555-6666). If you click or have your cursor in the second set of numbers (123-555-6666) it behaves odd but has the correct number of digits in place but is the wrong number. If you place your cursor in the last set of digits (123-555-6666) it not only has an incorrect number autofilled but it has an incorrect number of digits. I took a video of the behavior for reference.
With parenthesis in the number, trying to use autofill will not work at all anywhere. When taking the parenthesis out it works if nothing is entered or if you put your cursor in the first three digits before the first dash (123-555-6666). If you click or have your cursor in the second set of numbers (123-555-6666) it behaves odd but has the correct number of digits in place but is the wrong number. If you place your cursor in the last set of digits (123-555-6666) it not only has an incorrect number autofilled but it has an incorrect number of digits. I took a video of the behavior for reference.