Bug - Some app icons not showing up correctly
The window button panel plugin, alt tab cycling and other elements of Xfce are not using the correct icon from the chosen theme. This doesn't happen with every app but some apps like kitty, calibre, gnome-calculator, steam, pidgin, keepassxc, regular folders and potentially others are not using the theme's icon. Menus are using the correct icons though.
I asked on Xfce forums if anyone knew how to fix it and a moderador was able to replicate this issue and encouraged me to create a bug report.