That's why, you have 14 badges, once you got 13 badges the dens will update with new mons, & unique encouters as far as I know relocate to another den.
If my memory doesn't betray me, I recall encountered ditto in the den near Cairo's House, you know the guy who tells you about red essence & buy fragments, thats how I encounter mine from my V12 save file, except I fainted mine for 2 reasons, I didn't knew how to capture raid pokemon & I already have ditto from previous version so no biggie. Anyway try other dens is the den near Cairo's house is not the one, but I swear is in that one. So let me know if you encounter yours.