It is working for me with a RTX 4070 with Shotcut 24.04 on Windows including at 100%, which is automatic and not lossless! BTW, this automatic mode is an NVENC thing (decision-making), and I find the quality to be quite poor.
For lossless, see
But also, that creates a very large file when compared with x264 lossless; I discourage usage of h264_nvenc for lossless.