I have a freshly installed, completely stock Gnome installation and I can confirm this issue.
I just fully updated after installation maybe an hour ago and didn't encounter any problems, so this issue is brand new.
sudo pacman -S qgnomeplatform-qt5
[sudo] password for gnome:
resolving dependencies...
looking for conflicting packages...
Packages (2) adwaita-qt5-1.4.1-3 qgnomeplatform-qt5-0.8.4-2
Total Download Size: 0.35 MiB
Total Installed Size: 1.15 MiB
:: Proceed with installation? [Y/n] y
:: Retrieving packages...
adwaita-qt5-1.4.1-3-x86_64 210.5 KiB 1413 KiB/s 00:00 [--------------------------------------------------------] 100%
qgnomeplatform-qt5-0.8.4-2-x86_64 143.2 KiB 848 KiB/s 00:00 [--------------------------------------------------------] 100%
Total (2/2) 353.8 KiB 1646 KiB/s 00:00 [--------------------------------------------------------] 100%
(2/2) checking keys in keyring [--------------------------------------------------------] 100%
(2/2) checking package integrity [--------------------------------------------------------] 100%
(2/2) loading package files [--------------------------------------------------------] 100%
(2/2) checking for file conflicts [--------------------------------------------------------] 100%
error: failed to commit transaction (conflicting files)
adwaita-qt5: /usr/include/AdwaitaQt/adwaita.h exists in filesystem (owned by adwaita-qt)
adwaita-qt5: /usr/include/AdwaitaQt/adwaitacolors.h exists in filesystem (owned by adwaita-qt)
adwaita-qt5: /usr/include/AdwaitaQt/adwaitaqt_export.h exists in filesystem (owned by adwaita-qt)
adwaita-qt5: /usr/include/AdwaitaQt/adwaitarenderer.h exists in filesystem (owned by adwaita-qt)
adwaita-qt5: /usr/lib/cmake/AdwaitaQt/AdwaitaQtConfig.cmake exists in filesystem (owned by adwaita-qt)
adwaita-qt5: /usr/lib/cmake/AdwaitaQt/AdwaitaQtConfigVersion.cmake exists in filesystem (owned by adwaita-qt)
adwaita-qt5: /usr/lib/cmake/AdwaitaQt/AdwaitaQtTargets.cmake exists in filesystem (owned by adwaita-qt)
adwaita-qt5: /usr/lib/libadwaitaqt.so exists in filesystem (owned by adwaita-qt)
adwaita-qt5: /usr/lib/libadwaitaqt.so.1 exists in filesystem (owned by adwaita-qt)
adwaita-qt5: /usr/lib/libadwaitaqt.so.1.4.1 exists in filesystem (owned by adwaita-qt)
adwaita-qt5: /usr/lib/libadwaitaqtpriv.so exists in filesystem (owned by adwaita-qt)
adwaita-qt5: /usr/lib/libadwaitaqtpriv.so.1 exists in filesystem (owned by adwaita-qt)
adwaita-qt5: /usr/lib/libadwaitaqtpriv.so.1.4.1 exists in filesystem (owned by adwaita-qt)
adwaita-qt5: /usr/lib/pkgconfig/adwaita-qt.pc exists in filesystem (owned by adwaita-qt)
adwaita-qt5: /usr/lib/qt/plugins/styles/adwaita.so exists in filesystem (owned by adwaita-qt)
qgnomeplatform-qt5: /usr/lib/libqgnomeplatform.so exists in filesystem (owned by qgnomeplatform)
qgnomeplatform-qt5: /usr/lib/qt/plugins/platformthemes/libqgnomeplatformtheme.so exists in filesystem (owned by qgnomeplatform)
qgnomeplatform-qt5: /usr/lib/qt/plugins/wayland-decoration-client/libqgnomeplatformdecoration.so exists in filesystem (owned by qgnomeplatform)
Errors occurred, no packages were upgraded.