Ok, I've tested it with __tidy__ extension switched off. xdebug shows that the string (independent of its length, by the way) changes its value to unreadable charachters in line 677 of __lib-htmLawed.php__ saying (not clear yet for me):
$t = preg_replace(array('`(<\w[^>]*(?<!/)>)\s+`', '`\s+`', '`(<\w[^>]*(?<!/)>) `'), array(' $1', ' ', '$1'), preg_replace_callback(array('`(<(!\[CDATA\[))(.+?)(\]\]>)`sm', '`(<(!--))(.+?)(-->)`sm', '`(<(pre|script|textarea)[^>]*?>)(.+?)(</\2>)`sm'), 'hl_aux2', $t));