Comment # 12 on bug 1134475 from Stefan Dirsch

IIRC amdgpu proprietary driver installed a file below /etc/dracut.conf.d in
order to change the firmware file path (driver comes with his own firmware
files). The big issue was, that it did'nt uninstall this config file during
uninstall. So please make sure there is no such file left afterwards. Otherwise
there will be no firmware files in initrd after running mkinitrd.

BTW, there is a command to list initrd content.

sudo lsinitrd /boot/initrd..

so you can check whether the firmware files are generated to initrd.