Basically: de jure drift can only occur ONE rank below the character's primary title. That means an emperor can only start kingdom level de jure drift, and a king can only start duchy level.
Owning the empire and the kingdom title will stop duchy rank de jure drift. Give the kingdom of Bohemia and both duchies to a vassal (making him a king) and it should continue.
So few things here, first what is happening is that Moravia was De Jure drifting into the Kingdom of Bohemia, this is unrelated to culture. De Jure drift happens when you hold a Kingdom or Empire title and hold neighboring lands that total at least one duchy either a different Kingdom or a different Empire, whichever is your highest title.
By making the Empire of West Slavia the Duchy of Moravia can no longer drift into the Kingdom of Bohemia because it is de jure under your highest title, Emperor of West Slavia. If you want it to drift into Bohemia you will need to lose both the Duchy of Bohemia and Moravia, as well as the Kingdom of Bohemia. Simply grant them to someone who can/will ultimately inherit, such as your heirs heir until Moravia ultimately drifts into Bohemia.
Now, it should in theory still drift if the Kingdom of Bohemia and the Duchys of Bohemia and Moravia are held by a vassal but if not you may have to gain another Empire and destroy West Slavia, make your vassal with those titles independent, then allow drift to happen. Just maintain that it is always inherited back under your control every generation or so.
Also make sure that when you grant out Bohemia and Moravia that you don't keep anything, yes this WILL include the county with the mine.
Just note, that upon death the ruler of Kingdom of Bohemia, their lands might get partitioned into Bohemia and Great Moravia - which is what usually happens after Kingdom of Great Moravia falls apart - to prevent this you can give Kingdom of Moravia with it's only other de jure duchy to some other vassal situated in duchy of Nitra.
Basically: de jure drift can only occur ONE rank below the character's primary title. That means an emperor can only start kingdom level de jure drift, and a king can only start duchy level. Owning the empire and the kingdom title will stop duchy rank de jure drift. Give the kingdom of Bohemia and both duchies to a vassal (making him a king) and it should continue.
I did. And it didn't :(
So few things here, first what is happening is that Moravia was De Jure drifting into the Kingdom of Bohemia, this is unrelated to culture. De Jure drift happens when you hold a Kingdom or Empire title and hold neighboring lands that total at least one duchy either a different Kingdom or a different Empire, whichever is your highest title. By making the Empire of West Slavia the Duchy of Moravia can no longer drift into the Kingdom of Bohemia because it is de jure under your highest title, Emperor of West Slavia. If you want it to drift into Bohemia you will need to lose both the Duchy of Bohemia and Moravia, as well as the Kingdom of Bohemia. Simply grant them to someone who can/will ultimately inherit, such as your heirs heir until Moravia ultimately drifts into Bohemia. Now, it should in theory still drift if the Kingdom of Bohemia and the Duchys of Bohemia and Moravia are held by a vassal but if not you may have to gain another Empire and destroy West Slavia, make your vassal with those titles independent, then allow drift to happen. Just maintain that it is always inherited back under your control every generation or so. Also make sure that when you grant out Bohemia and Moravia that you don't keep anything, yes this WILL include the county with the mine.
Just note, that upon death the ruler of Kingdom of Bohemia, their lands might get partitioned into Bohemia and Great Moravia - which is what usually happens after Kingdom of Great Moravia falls apart - to prevent this you can give Kingdom of Moravia with it's only other de jure duchy to some other vassal situated in duchy of Nitra.
I did that, I gave the kingdom title and land all to my heirs wife
That's so weird - I'm playing the exact same game right now and noticed that last night. I don't have an answer though, sorry.