If you put a square wave through an RIAA network surely that is what you'd expect?
Otherwise, what exactly do you think it should be doing with the high frequency component of the waveform? If it let it through unaltered it wouldn't be much use in eq'ing the output from a cartridge playing a record.
Here's an Aurorasound Vida reproducing a 1 KHz sine wave.
and here is the same phono stage applying RIAA correction to a 1 KHz square wave
That's what it's supposed to look like. You need to remember what a square wave actually is.