Noname, a square wave is not a good merit to justify that the mod does nothing.
After the modification the analog BW rolloff is extended to around 10MHz…
In other words, the effective sampling rate of the quad as well as the analog BW of the front end is not good enough to accurately display a square wave at these frequencies. The “sharp edges” of a square wave (higher harmonics) will be attenuated and you will end up mostly with the fundamental.
If you want to be looking at a fully featured square wave you would probably need a scope with way higher analog BW and sampling rate.