I have 8.1 Beta 1 (Mac), but the problem was occurring in the released version too. It occurs when the library is sorted by a column that groups the results (type, author, series, etc). Clicking on the triangle twisty control works properly, but clicking anywhere else in the header line of a group makes the triangle twisty control disappear.
The disappearing triangle is clearly a bug of some sort, but it seems to me that clicking anywhere in that line (not just the triangle) should open and close the group section, but it is not. I feel like that is how it used to work in L7, but I can't be sure.
In this instance I clicked on the word "Bible" (before and after):

