Hmm!
So for me, when I leave my iPhone X on it, it flashes red all the time (LED at the base) and it charges normally!
Does your S8 recognise that it is charging or does it just flash and S8 does not charge?
Yes, the iPhone X is charged - but not with the full possible speed of 10W, as Apple does not support this. With an upcoming update, 7.5W should be enabled (currently charging at 5W).
Greetings