I patched the problem this was:

function history_log_event_direct( $p_bug_id, $p_field_name, $p_old_value, $p_new_value, $p_user_id = null, $p_type = 0 ) {

Only log events that change the value

if( $p_new_value != $p_old_value ) {
if( null === $p_user_id ) {
$p_user_id = auth_get_current_user_id();
}

$c_field_name = $p_field_name;
$c_old_value = ( is_null( $p_old_value ) ? '' : $p_old_value );
$c_new_value = ( is_null( $p_new_value ) ? '' : $p_new_value );
$c_bug_id = db_prepare_int( $p_bug_id );
$c_user_id = db_prepare_int( $p_user_id );
$c_type = db_prepare_int( $p_type );

#patch:
if ( strlen($c_old_value)>255 ) {
$c_old_value = substr($c_old_value,0,250) . "...";
}
if ( strlen($c_new_value)>255 ) {
$c_new_value = substr($c_new_value,0,250) . "...";
}
#end patch

$t_mantis_bug_history_table = db_get_table( 'mantis_bug_history_table' );

$query = "INSERT INTO $t_mantis_bug_history_table
( user_id, bug_id, date_modified, field_name, old_value, new_value, type )
VALUES
( " . db_param() . ', ' . db_param() . ', ' . db_param() . ', ' . db_param() . ', ' . db_param() . ', ' . db_param() . ', ' . db_param() . ' )';

$result = db_query_bound( $query, Array( $c_user_id, $c_bug_id, db_now(), $c_field_name, $c_old_value, $c_new_value, $c_type ) );
}
}