I’m trying to configure Shotcut to use the av1_nvenc codec to create my proxies. I have Settings → Proxy → Use hardware encoder checked and also after clicking Configure Hardware Encoder I can see it detected 3 codecs (av1_nvenc, h264_nvenc, hevc_nvenc).
For this test I just go to a video and manually click Make Proxy from properties panel.
Even if I disable the other 2 and leave av1_nvenc checked, the resulting file is using x264 codec.