I did some tests and it is indeed a bug, but another kind than you would probably expect. The intended behavior in this case is actually an error message "Selected layers cannot be merged without changing the image." instead of the blank layer.
The Silhouette style makes it impossible to merge just the two layers and maintain the overall look. The Silhouette interacts with every layer beneath it and if it is removed, the effect is lost.
Perhaps it would be better to ask the user if they want to perform the merge anyway in this case.
(It works if there are only 2 layers, because in that case, it can be performed and the whole image stays the same. If the bug wasn't there it would also work if you moved the 2 layers all the way down and then merged.)