Byte Enables are not supported in this configuration due to the way that the memory gets implemented.
The widest port width supported in True Dual-Port Mode for a single M9K block is x18. This means that a 32x8 bit memory actually gets implemented as two separate 16x4 bit M9K blocks. Although in this scenario, each M9K block maintains it's 4:1 ratio, 4 bit wide byte enables are not supported.
Byte enables are only supported for True Dual-Port memory configurations when both the PortA and PortB data widths of the individual M9K memory blocks are multiples of 8 or 9 bits.