I have mounted three network folders (fstab) on my test environment (dual boot with zorin16). In Zorin, these are displayed correctly in all programs and files can be opened or music played via them.
In KDE Neon the folders are displayed in Dolphin, but in Writer or Rhythmbox the folders are not present. Probably also in other programs. in Okular it works.
So I can't use KDE Neon productively. Too bad

My System:
KDE Neon
KDE-Plasma: 5.23.4
KDE-Frameworks: 5.89.0
QT-Version: 5.15.3
Kernel: 5.11.0-43 (64bit)
Grafikplattform: X11

Hello marcellot !

The differences between dialog boxes (like Open file) and what they display are due to the differences between GTK (Gnome) and Qt (KDE) apps.

Rhythmbox is mainly a Gnome application so this one uses GTK libraries to display dialog boxes.
As I don't use Rhythmbox, I can't help you further, but I would recommend you to try Clementine instead.

By default LibreOffice too, but you can install specific packages (libreoffice-kf5 or libreoffice-kde5 or libreoffice-qt5, may be different on some Linux distributions) to get it more integrated. I use KDE Neon, I installed libreoffice-qt5, libreoffice-kf5 and also libreoffice-plasma*, and works fine.
But I don't use LibreOffice from Flatpack, only the version (yet 6.4.7) from ubuntu repositories (with Synaptic not Discover)

In fact, dialog boxes may change on other applications, this also happens with Java applications (even under Windows). It's something you'll have to deal with, what ever the environment or distribution you use.



* libreoffice-plasma was already installed, but still have GTK dialog boxes

What Chtiland (north France I guess ) says. Gtk and qt apps. Hit or miss.

@chtiland: Thanks, I understand the problem, but that don't help me.
With mint or zorin none of the problems appear.
My decision is clear. I stay there.
Thanks