Rob, are you doing Autofit and Number cell formatting, or just Autofit?
I tried this out and saw the issue with numbers greater than 11
digits. Like you said, changing the width didn't help. But changing
the format to Number (with 0 decimal places) did.

Playing around a bit, I also found that numbers 1E16 have truncated
precision, so you may want to watch out for that if you're
concatenating very large numbers