I think we have a problem with the build environment:
$ ldd /usr/lib/libvulkan_radeon.so
linux-vdso.so.1 (0x00007f615b27d000)
libLLVM.so.21.1 => /usr/lib/libLLVM.so.21.1 (0x00007f6150800000)
libdrm_amdgpu.so.1 => /usr/lib/libdrm_amdgpu.so.1 (0x00007f615b240000)
[...]
I removed vulkan-radeon and reinstalled to make sure I was using the one in obextra, but to solve the problem I still needed to install llvm21-libs as you suggested earlier because the packages in obextra were built against the earlier version.
I need to think of a way to signal this may be a problem at upgrade time, but for now the problem is solved for me, thanks for the suggestion.