I'm not sure if that's a bug, but grenades flying through a high mountain seems a little bit unnatural. :P I think screenshot will show everything needed.
http://i.imgur.com/wnioXH5.jpg
(See that little yellow dot in the center of image, that's our flying nade)
By the way in the same battle I found out that bow doesn't really require both hands to use. My brave Lumi without left arm and right thumb, shooted with a bow quite well. :P
hes throwing over what probably isnt a mountain, hover the cells there with your mouse and see what roof type it is, I doubt its mountain that close to the edge.
Yeah, there was a "thin roof" in that place. I've run some tests and it's really hard to tell if grenadiers are able to throw over "mountain roof" too, because when using "destroy" in dev mode, it destroys also mountain, and makes "thin roof". But anyway my colonist are not able to throw it over, so I don't think that's an intended feature.
http://i.imgur.com/i6SNlvP.jpg
It's working okay. Projectiles can go 'through/over' things in a few circumstances, including this. Thanks for reporting.