Photos are cleared when selecting a different collection in the Print Module

In the Lightrooom Classic "Print" module, I can't seem to add photos from different collections.

I add cells to a "Custom Package".  Then I drag a photo from the filmstrip to one of the cells.  I want to add a photo from a different collection, but when I click on a different collection, the photo that I had previously added disappears.

Is this how it is supposed to work?