We have a 2021 FC 25 Twin Bed and indeed it slides. It was easy to fix. The foundation is just a piece of plywood. It probably took me an hour to fix it, and most of that was just making sure I got it right ("measure twice" etc.) Now my mattress does not slide and I can get a good night's sleep. I heartily recommend this very easy fix.
And no, it doesn't reduce the amount of room between the twins appreciably.
I did add a piece of 1/4" birch plywood to one side and will be doing the other side. Adding the piece allows a blanket to be tucked under the mattress much easier giving a cleaner look it also keeps the mattress from drooping.
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