informationpanelcontent.cpp currently has an explicit check for whether or not a URL has a Places panel entry, and if it does, no folder preview is generated. It's not clear why this is done, but it was reported as a bug: https://bugs.kde.org/show_bug.cgi?id=392621. Removing the logic fixes the bug.
BUG: 392621
FIXED-IN: 18.08.0