Thank you for posting the video showing how the Arm Bottom broke.
I don’t know if it is helpful, but the Arm Bottom that I received from Niryo in my v2.0 kit weighed 85g. I suppose different filaments might have different weight and other properties but maybe that can serve as a reference point for the parts that you are printing.
I hope that the 40% fill part holds up better.
One more question: how are you embedding YouTube videos in your posts? When I tried to paste the EMBED text from Youtube into my thread in the Showcase section, I got a security error. Niryo’s WordPress functionality doesn’t seem to like something in that text.