Since we upgraded to 3.1.3, Behat started failing with this error: Notice: iconv(): Detected an illegal character in input string at /path/to/lib/configonlylib.php: 103

That is referencing this line in the min_fix_utf8 function:

$buggyiconv = (!function_exists('iconv') or iconv('UTF-8', 'UTF-8//IGNORE', '100'.chr(130).'€') !== '100€');

My guess is that we are seeing this now due to a change in error handling for Behat, introduced in MDL-46514.

From what I could tell, there is nothing you can do to avoid the buggy iconv except for modifying your PHP version. In that case, would it make sense to apply the same fix made to fix_utf8 in MDL-43026 (commit) to min_fix_utf8?