Hello Tim,
Thank you so much for the response!
Here are some screen shot of how the default map looks in my PC.
about Q1,
I had noticed when I opening the map, there is an atmosphere fog actor by default,
you may able to reproduce this problem by delete the ‘original’ fog actor and drag a new [atmosphericFog] into the map.
The map might looks like this
about Q2,
I had tried to add an exponential height fog into my defaul map.
this is how it looks like (I have hide the skysphere to demonstrate it better)
and this is another screen shot with the BP skysphere on
Another concern I had is, will this issue possibly due to my graphic card or any hardware issue (me not with good knowledge on hardware, excuse me if that sounds strange o.,O)
the graphic card on my pc is a NVDIA GeForce GTX760.
I hope those information helps.
Thank you once again and have a lovely holiday !