I don't know how exactly it works but I've noticed that there are three different window screens that can appear when opening a file from a QT based app.
I'm talking about the small window that pops out when we want to open a file from a specific program.
On most programs, we go to the File Tab on the upper Menu Bar and then click on Open File.
There are three types of windows that will show up depending on the program currently using.

1) Best Pop-Up Window:
This window allows us to use the Show Aside Preview, View Hidden Files, and the Shortcut Paths are customizable within the window. If we Add An Entry on Dolphin, it will appear on all QT-based programs using this particular window style. Very handy.
##This window will open when using: Gwenview, KolourPaint, Kwrite, VLC, and Krita.

2) Basic QT5 Pop-Up Window:
This window style doesn't allow us to use Show Aside Preview, show hidden files, and we can't change the Shortcut Paths. I don't know exactly how this window decided to use some of my manually made directories as shortcut paths.
##This window will open when using: Kvantum, Kid3, LMMS 1.2

3) Basic QT4 Pop-Up Window:
This window behaves exactly the same as the previous one only that this is for QT4 based apps.
##This window will open when using: PuddleTag, LMMS 1.1

############UPDATE#############
After browsing around i found out that I can edit path entries for Basic QT5 Pop-Up windows by editing file QtProject.conf
/home/USER/.config/QtProject.conf
This will only work for QT5 apps but not for QT4 apps. I've looked around and I don't see any file. I looked inside PuddleTag and nothing.

Also, this problem happens in Kubuntu 18.04 and Debian 10. On OpenSuse, Puddletag uses what I call Best Pop-Up Window. I don't know if OpenSuse developers fixed this issue or if this is something that was never truly fixed on Debian/Ubuntu.

##UPDATE##

The file /home/USER/.config/Trolltech.conf is responsible for the Basic OpenFile Dialog on QT4 apps. I tried editing the file but doesn't work either.

Again I'm having this issue on Debian/Ubuntu.

Did you try to drag a folder from the right panel to the panel with shortcut entries (url sidebar) ?