Can’t reproduce. Perhaps there are other parameters, which you don’t mention.
Do the following: Create a new site with only one page with only those 2 elements. Upload this .muse file to dropbox or a similar file sharing service and provide the download link here. Then we can have a closer look.
Thus, time consuming „question and answer“ games or pure guessing can be considerably reduced.
Thanks Günter. I've since built the rectangles into the master page, and that has resolved the issue (no longer have access to the state the muse file was in when the problem persisted, but thanks for offering the inspect the file).
I did, however, notice that the rectangles – when created and placed – did not expand the height of the page. Clearly this is related, but is beyond my understanding how a shape can be placed on a (regular, no master-) page without influencing the usual behaviour of automatically resizing (vertically).
This simply happens, if the object is defined as a "footer" element. This happens automatically, when you start drawing the object (inadvertently) in the footer area.