OS: Debian Sid amd64
Since 1.4.0 some UI elements are not themed when the application starts. I use QT_QPA_PLATFORMTHEME=gtk2 to match the GTK theme. Switching themes in Gnome Tweaks fixes the problem for the currently running instances. Other Qt applications are unaffected. Tested qBittorrent, SMPlayer and Audacious.
Effect: UI becomes visually unappealing and difficult to use.
To reproduce:
1. Start XnView MP with "env QT_QPA_PLATFORMTHEME=gtk2 xnview"
Actual behaviour (bug):
After starting:

After changing themes in Gnome Tweaks:

Expected behaviour:
The application immediately looks like the second picture.