I think your culprit is here:

WHERE `Keys`.`Keywords` LIKE '%$search%'
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sprintf is trying to interpret the % characters as format string tokens. If the first character of $search is a valid field specifier, you get weird results. You want literal % characters in the finished expression, so use '%%' instead:

WHERE `Keys`.`Keywords` LIKE '%%$search%%'
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...and I bet things might go your way.

Proof of concept:

bash$ perl -e '$a=22; printf "%$a%\n";'
                     %
bash$ perl -e '$a=22; printf "%%$a%%\n";'
%22%
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