This is a dangerous, buggy assumption: "provided we don't use thousands separator for the first long-type field (that shouldn't happen as Long.toString method is used in AbstractProblemStatistic.writeCsvStatisticFile to obtain the value)", because a new statistic might output any string that contains a comma. For example, suppose there is statistic which outputs a Move.toString() (surrounded by "), which might contain a List.toString() such as [1, 4, 5].

Fixed locally :)