You really are the only one this has ever happened to. I even tried creating a new site and installing Akeeba Backup 9.8.1 afresh. I can't reproduce your issue. If I let it use an auto-generated secret word there's never a complaint. This is also something I'm doing fairly regularly (several times every week) as I am setting up temporary sites to test features in development, or try to reproduce reported issues. Between that and the number of clients we have, I believe that if it was a bug we'd have definitely run into it over all these years.
The only case I can think that this might not be pure random chance is if PHP's regular expressions are subtly broken on your server, somehow not recognising underscore as valid punctuation. I have never seen that before, that's why I am hesitant to even entertain it as a likely explanation.
Nicholas K. Dionysopoulos
Lead Developer and Director
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