Soft floor discussion (pics and steps to fix included)

It appears from the video that you have a loose ribbon or stringer near the door (where the cabinet used to be). A joint may be cut in that area. I believe once you get the plywood up, you'll see where you need to attach support for the steps.

Typically, mfg's use an adhesive that doesn't interact with foam to bond the plywood to the foam and any joists below the plywood.