The policy changed early this year I believe. It certainly was in effect in February when I flew HKG-BKK-CEI. All passengers now clear customs at the first port of entry. Passengers connecting to other Intl airports in Thailand (HKT, CNX, CEI, USM, and KBV I believe) will remain airside to clear customs in BKK and pick up their bags at the Intl baggage carousel at the final destination.