So I'm well aware that the Custom Layout Update changed with update 2.3.4 so that you now need to create an XML file.
When Single Store Mode is switched to YES, the file is not visible in the Category admin menu under Design: Custom Layout Update.
Switch Single Store Mode to NO, and the file is then able to be selected in Design: Custom Layout Update.
I am using the Olegnax Athlete 2 theme and the theme developer assures me there is nothing wrong with the theme. I'm including two screenshots of the stores config in Single Store mode below.
Notice in single store mode the store and store view are blank in the Content:design:configuration. I believe this is normal in single Store mode.
Any help would be appreciated.