SSM2604 CODEC DAC Output filter response

I am using an SSM2604 at 64 kHz with a 12.288 MHz MCLK. When outputting signals above fs/4 I see increasing imaging products. At 20 kHz the output waveform looks like linearly interpolated discrete points. The data sheet shows a DAC digital filter frequency response that has around 60dB attenuation for signals above 0.55fs. I assumed this meant that images would be attenuated by the amount indicated.

Am I doing something wrong? Is the DAC digital filter just linearly interpolating between values instead of interpolating in a bandwidth limited fashion? Is a separate antialaising filter required? Or does the DAC digital filter just apply to noise & not images?

The is little information available about this part in either the data sheet or else where on the web site.