can be either genunchi or genunche in both singular and plural, but the other forms depend on that choice which makes this noun either masculin or neuter, see
https://dexonline.ro/definitie/genunchi#paradigmTab (tab declinări)
Good observation, but everyone uses the masculine noun with identical singular and plural forms: un genunchi, doi genunchi.
Many people would even tell you "genunche" is a mistake (in either singular or plural). There was a notorious case when the Romanian minister of Education posted an update on his health situation on social media which went like this: Totul este sub control, am ieşit din operaţia la genunche. Needless to say, he has been mocked ever since and dubbed ministrul "genunche".
The thing is, dexonline is a complete reference, but it may not always give you a gist of the modern/ common language. I recommend using this dictionary when you are not sure about the right or preferred word form or paradigm. This is the ultimate book on correct writing in modern Romanian.
Thank you! Here that link again, for those hyperlinks do not work in the (Android?) app:
(and since you do German: das Knie (Sing), die Kni-e (Plur) is spelled the same but is pronounced (by most Germans) differently - it is a confusing word as to Sing./Plur. in German, too...)