I still have this problem in 9.2.1-RELEASE:
I have ten 2TB Seagate drives running at present -- eight connected to an M1015 in IT mode, and two connected to the motherboard SAS ports (flashed to IT mode) of the X10SL7-F.
Only four serial numbers display in the GUI: three of the eight connected to the M1015, and one of the two connected to the motherboard.
I adapted a shell script I found online to display the temperature and serial number for each drive, and it shows all the serial numbers.