I just tried reproducing this using the same example content but I see the translucency vertex fog working as expected.
I made a new level and placed the BP and then placed both atmospheric and exponential fog separately and both worked. You can see its vertex based by the vertex interpolation issue in this image:
I can only suggest making a new test level and placing only the water BP and fog first. Then verify the vertex fog setting is correct. Then possibly try placing a regular mesh without using the BP if that still doesn’t work.