I'm not sure if I understand the question correctly, but it seems that it worked before? Do you have this problem for example on your production environment, or your local environment. What did you do that it didn't work anymore?
I guess you can try to add the field again in the custom page properties. I think that for some reason your .config file isn't updated or deployed, so you miss the field inside the UI. If you add the field again, I think that the data will show up.
Let me know how that works for you?