is there a defined extent, clip applied, or other reason that was applied to the "existing" features, but you are adding a feature outside of that? Is the feature available in the attribute table just not visible in the map? Can you use the zoom to it, proper location, by using the attribute table? Perhaps try changing symbol, or putting that feature in a different class of symbol just to see if it shows up? Not knowing your full setup, I think the first step is to validate Pro is even recognizing it. Simple question (sometimes we forget simple) - did you save the feature? And can you do the reverse - add it in Pro and see it just fine in ArcMap?
