I have been in the mechanical work force for over forty years now and this is the first time I have observed a check valve in a drain line.
This line has a stand pipe and P-trap on the other side of a concrete wall in a full basement. I think I understand why it was placed in this position, to prevent sewer from backing up and coming out of the standpipe as it is the lowest open drain in this system. Makes me think this home has had past blockage in the main drain line. I guess I am going to have to check the IRC mechanical as I have never ran into this before just a shade tree plumber.
I did discharge water down this drain and the check valve did open and the washing machine has been in use as the home was occupied. What say YOU