As Tony stated, it works as intended, so not a bug. That is if Carex leporina is more often reported than the ‘true’ C. brizoides, it will appear on top, even if typing the synonym. (Not anymore now as the synonym for C. leporina has been deleted)
Just want to mention that other users got confused by this and wrong entries have been made.
Also note that the screenshot with the synonym in brackets is from the website. In the Android app, it is not displayed