74HC4051 use problem

Using 74HC4051, the eight inputs are connected together to a sine signal (only for testing so the inputs are connected together), and the sine signal measured at the output is as shown in the figure, the waveform is very wide, about 200mv. And when measured with an oscilloscope pen, it sometimes turns into a normal waveform by itself as shown in the figure below

The above phenomenon feels like a chance event, which happens occasionally.

And the experiment before obviously did not have this phenomenon, it was always a normal waveform. I added a bypass capacitor coupling capacitor to the op-amp in other parts of the circuit, and then it became like this.

Does anyone know what is the reason for this?