Steven, I've found a hint, and a workaround.
While trying to find common features of the problem albums, I noticed that each concerned folder contains a subfolder named (DOC), storing some documentation about the recordings/works/artists.
After moving those (DOC) folders to other locations and refreshing, the problem ceased. In the same stride, I created a (DOC) subfolder in another album folder, => that album lost its cover, too. However, removing the parentheses from the subfolder name stopped the problem.
Thus, so far, the bug seems to show up if there is a subfolder name STARTING with '(' or ')', whereas there is nothing wrong with parentheses elsewhere in the name (middle, end). Moreover, the bug wasn't present in former 3.4/3.5 builds.
A peculiar case, isn't it?