Well, first off, it seems a hearty welcome to the forum is in order!
There are, at the moment, some bugs that the developers are working on. This is a known problem, that I too have encountered.
I haven't been able to put my finger on it, but from my experience, it seems as if this problem only persists in slopes from one specific direction. For example, when I build a symmetric structure with a sloped roof, one side of this roof is affected upon save/exit/reload, and the other is not.
EDIT: I believe this should be a bug that users and developers should easily be able to reproduce, as, for me, it has occurred in every single character, in every single world, in every single structure using these smooth slopes.