An update came down a few days ago and now all the folders have miniatures enabled except for those which have the entry [Dolphin] PreviewsShown=false in their .directory file -- KDE Neon Developer-Unstable Dolphin version: 18.03.70 KDE Frameworks version: 5.43.0, Qt version: 5.9.3
This behavior is the new default for Dolphin, please check: https://bugs.kde.org/show_bug.cgi?id=338492
Ok, thanks