You normally can't use "dumb" optical triggering for off camera HSS/Auto FP flash
Using S1 will very likely disable HSS/Auto FP on the off-camera flash. This is because HSS/Auto FP requires more sophisticated communication between the camera and flash than a simple "dumb" optical flash to sync precisely enough.
You haven't told us what you are using to trigger the Godox 860IIN, but any form of optical triggering will probably not work for HSS/Auto FP, particularly for very fast shutter speeds such as 1/8000 seconds. The only way I've ever seen HSS work with an off-camera flash is via an HSS capable radio trigger or a hardwired connection.
By the time your trigger flash has fired and the slave has responded to the pulse of light and fired it's all over but the crying as far as the shutter transit is concerned. Your primary (trigger) flash will be synced with the first shutter curtain. But with shutter times shorter than the camera's X-sync speed, the second curtain will already be almost closed as well by the time the first curtain is fully open to send the "fire" signal. Then the circuitry in the slave flash will take a few milliseconds to fire the slave flash. By which time the shutter will likely be completely closed.
Even when the camera and flash are syncing properly
You are using a shutter time of 1/8000 second. Thus, you need to use High Speed Sync/Auto FP to allow the full frame to be illuminated by sequential pulses of the flash as the very narrow slit between the first and second shutter curtains transits the sensor. The faster the shutter speed, the narrower the slit will be as the shutter curtains use the same speed to transit in front of the sensor.
The flash must pulse very rapidly to do this. This reduces the overall power of the flash with regards to the amount of light that actually makes it to the sensor. Most of the output of the flash that makes it down the lens to the camera's light box is striking the front of one of the shutter curtains as they transit in front of the sensor. Only a small percentage is actually striking the narrow strip of the sensor that is exposed between the curtains when each pulse of the flash reaches the camera. Remember, the faster the shutter speed, the narrower the slit between the first and second curtains.
Flashes have limited power. How much power they have is determined by how much energy can be stored in their capacitor(s) and how much heat the system can tolerate without being damaged when that energy is released. Many speedlights limit the amount of total output power of the flash when using HSS/Auto FP to prevent overheating. At most, the total amount of power used by many speedlights for all pulses combined will only be about one-half of the total amount of power released by the same flash with a conventional single dump at full power.
When one uses HSS/FP, the faster the shutter time used, the narrower the slit is between the two chutter curtains. Thus, if the flash is outputting the same total amount of light divided over a higher number of pulses, less of it is making it past the shutter curtains and striking the sensor.
- The faster the shutter time, the narrower the slit.
- The narrower the slit, the lower the percentage of light from each pulse of the flash will be striking the sensor compared to the percentage of light that will be striking the front of the shutter curtains.
- The faster the shutter time, the more pulses the flash must make as the shutter curtains transit the sensor.
- The fixed amount of total power available must be divided between each pulse. This means that for the higher number of pulses used at shorter shutter speeds, the weaker each individual pulse will be compared to the fewer number of pulses needed for a slower shutter speed.
- The lower percentage of the flash's output that is actually striking the exposed sensor, combined with the the lower amount of power used for each pulse, means the effect the flash power will have on your shot will be less even though the flash is outputting the same amount of total energy.
Assuming your camera and flash are correctly syncing, you probably are getting as much power as can be expected in the shot. But at 1/8000 second that won't be very noticeable if the ambient light is very bright.
You can test it by using a dark room with no ambient light. If you have a highly reflective object, such as jogging apparel with high visibility reflective 'safety stripes' on it, place it in the field of view. Take frames both with and without the flash turned on. The difference should be obvious.
If there's no difference, then you probably have a syncing issue.