The Iron Curtain is throwing up several curveballs at the moment which are very confusing as the process I follow is pretty much the same as for the FM20 version.
For the Historical Divergence it means I will from now on not re-use competitions from other nations, even if it is the most logical thing to do. You have to create from scratch, because making a duplicate didn't work for me for some reason.
Some testing the last few days has shown that the issue is the competition and not the teams itself. Slovak teams in a Czech competition, no issue. Czech teams in a Czech competition, no issue. Slovak teams in a (formerly) Slovak competition, issue. Slovak teams in a newly created competition, no issue. At least for the builds I have right now, they are first level competition of the nations I am building, so no competitions have been moved to other nations.