As title says - macOS 10.15.2 (Catalina) and the latest versions of Safari (13.0.4 production & 13.2 Tech Preview) have the same behavior. Chrome behaves as expected, as does Opera. Didn't check Firefox.
When you find a layer that you read its description etc. and want to add it to your map in the Map Viewer Beta there is no Add to map button at the bottom. Took me a while, but it is there ... eventually. You have to scroll about 3 pages worth of empty grey after the white text background stops and you'll get to the button. I tried multiple different data sets and all did the same thing.