ok i think i figured out how to fix this.
i had the same problem and looked at chapter select and noticed that one chapter says its only been completed on hard (not true). so i completed that chapter again on crushing. after it definitely says that all chapters are completed on crushing difficulty, i played the final mission as well as the epilogue and the trophy popped at the end credits! i think you only need to complete the epilogue so everyone DO THIS!!! if it works then awesome! if it doesnt then im sorry but it worked for me.
I also did some of my own digging. I went into chapter select and scrolled through all of the chapters, noting all of them said "crushing" ....except one!!!! I have NO idea how or why this particular chapter switched to "light" mode...but there it was. So, I am in the process of completing that chapter on crushing...with luck that will pop the **bleep** trophy.
Will update when I complete it tomorrow! Hope this helps if you haven't checked there first before doing another crushing run!