Hi - It looks like Kamoso is still using KDE Frameworks 5, so one catch here is that the relevant Arch Linux package is actually kirigami2. However, that should have been installed already as a dependency of the Kamoso package.
I won’t pretend to fully understand why, but from searching around changes to Kamoso’s packaging elsewhere - snapcraft: set qqc style to desktop. (!21) · Merge requests · Multimedia / Kamoso · GitLab - it looks like there are some cases in which Kamoso malfunctions if the environment variable QT_QUICK_CONTROLS_STYLE is set with a value other than org.kde.desktop (seen before with some other apps - [SOLVED] Plasma 6 System Settings not launching / Pacman & Package Upgrade Issues / Arch Linux Forums)
Trying to run Kamoso with that environment variable set might be worth a try?