We also thought so, but even if we set the Sun Multiplier to 0, at first glance everything looks OK, but if you fly into the clouds, they are still transparent.
There seems to be a problem in the material - trueSKYTransparency and modifier or in C++.
On Thursday, I wrote an e-mail with a bug report, since then new versions of 4.1a (ue4.20) have stopped coming out. So we will wait and hope that is not a coincidence.
If you look closely, there is a difference between off and on, but not the same as before.