There is noise in the switching power supply transformer. When measuring the optocoupler, the noise disappears immediately after the probe is lapped on pin 1, and no problem is found. Very confused.
as you said, the noise disappears as soon as the stylus is put on pin 1. That may be caused by parasitic vibration. You can use an insulated small wire with an appropriate length (just weld the small wire like a radio) and put one end on pin 1. If the wire is as short as possible, it can suppress the noise and make it disappear. You can weld the small wire in a small circle with a screwdriver to pin 1.