This is not a bug. The late rounds of the national tournament are important games, just like playoff games. Don't expect your players to take them too easy!
Regarding the enthusiasm management, Playoff normal gives the same boost to enthusiasm as Take it easy while the effort is equal to Normal. On the other hand, Playoff crunch time is the same as Normal in terms of enthusiasm, while your players play like on Crunch time.
These things are actually explained in the Game manual.