Hidden iFrame Not Loading

I have a form with a couple of Master/Detail apps as iFrames (on is a grid and one is a form). Each iFrame is in its own block, and for design reasons I need to start with one block hidden and one block showing, with buttons to toggle block visibility. Which block is set to be hidden at start is determined onLoad using the sc_block_display() macro.

The issue is that whichever iFrame in the initially hidden block doesn’t end up loading at all. Even if I then set the block to visible, it remains completely blank. Moreover, when I look at the HTML, the contents of the iFrame have been replaced with something like the following

<iframe class="css_iframes" id="nmsc_iframe_liga_A_grid_ivsdetails" marginwidth="0px" marginheight="0px" frameborder="0" valign="top" height="0px" width="0px" name="nm_iframe_liga_A_grid_ivsdetails" scrolling="auto" src="NM_Blank_Page.htm"></iframe>

Has anybony else encountered this before? Is there a workaround?