You need to visit "Configuration -> Attributes" page. Find "memberOf" attribute in the list there, open it for edit and make sure properties marked as green on the picture I've attached to this post is set accordingly. You must set "Usage type" as such for attributes you need to be available for OIDC workflows, and you also must assign some "oxAuth claim name" to them. Until I'd done that for memberOf I also couldn't get it requested by RP.
But, still, it seems there is another issue. I only able to see one entry of my new member_of claim in the output of test python script. I'm still unsure whether it's a problem of Gluu, of the script itself, of mod_auth_openidc. Marek, could you check for yourself and let us know whether you able to receive both enties of memberOf at RP?