I've managed to get 6 antediluvian dwarven map fragments on a fresh 2.3.1 game. Reading any of them still gives the "Sadly important parts are missing so that you can't make sense of it." message. As far as I know, there are only 6 fragments, so there shouldn't be anything "missing".

I've tried various other things (talking with various NPCs, giving the fragments to various NPCs, making sure all the fragments are the same status, reading scrolls of repair, ...), and so far nothing works.