Just found a way to fix it!
It requires a Xbox Series X or PS5 and Witcher 3 4.0 on your Xbox One or PS4 (for cross saves).
Just before you go in the hideout, make a manual cross-save on your PS4 or Xbox One.
Then, switch to your Xbox Series X or PS5 and open the cross save.
When you open the chest in the hideout on your Series X or PS5, the plans will be there!
I tested this myself on PS4 Pro and Xbox Series X, but from what I've read on Reddit, it also works with PS5. You can also apparently transfer the save game back to your last-gen console and use it with the plans.