You’ll need to cut out the soft areas from the bottom. Either replace with foam board, and plywood, or just a thicker piece of plywood and forget the foam board.
But you have perimeter support that was mentioned above, plus the wall has a bottom 2x2 support that runs the entire length so it’s bridging over the soft areas.
It’s best to try and figure out where the moisture came from, so it doesn’t happen again.
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