In the screenshots you posted I don’t see where you select 10bit. How do you know it’s being processed as 10bit?
It could still be processing it as 8bit but then saving it in a 10bit ProRes file. It would only be 8bit info but in a 10bit wrapper.
That’s why I prefer command line because you have complete control of the workflow.
If it outputs a ffmpeg log file you can confirm its workflow with that.