Why is extrude just moving face instead of expanding it? I have a model like this, planes, surrounded by cubes/


I want to extrude all the faces up to give it a little extra thickness before exporting to Zbrush like this

but when I make the extrusion, when I flip it back over, all of the faces on the top have just been moved, not extruded, which is not what I want. The cubes all extrude down, but the planes do not.

Any idea why it is doing this? Any idea how I can make it not do that?
I found this answer to a related post How to extrude and leave the original in place?
Select the Face, Shift+D (duplicate), cancel (Esc), extrude (E)
then select everything and do a Merge by Distance in order to remove the duplicated verts.