Changed /system to f2fs, cannot change back to ext4

I changed /system partition to f2fs for using temasek. Now I want to change it back to ext4. I converted all the other partitions to ext4 successfully. But when I try to change /system to ext4, it appears to go successfully. However, when I reboot, it still shows f2fs in the TWRP recovery.
I am using latest TWRP recovery (2.8.7.0_7 I believe).
Can anyone help me on this?