By default the first model is ‘selected’, but the builder can’t find the sObject. As a workaround, you can select a different model, and then select the first one again, and *poof*, sObject found. But that doesn’t help on pages with one model.
That is a strange one. A workaround is to choose the second model and then go back to the first to force the metadata load. But if you only have one model on your page…
I’ll document the bug. Thanks.
Yep. I had to create a second model on a page just to accomplish the workaround. 
