The code handling clean shutdown with a version downgrade currently disables log archiving in WiredTiger - that causes WiredTiger to leave log files present in multiple different formats, which will lead to issues in terms of checking version compatibility when starting up databases with earlier versions of MongoDB.
The particular code in question is in WiredTigerKVEngine::cleanShutdown
The archive=false setting has been passed since the code was added. There was a conversation in the code review process, which indicates that was done to enable manual checking during development.
I think it's better to allow WiredTiger to remove the previous version log files when downgrading, so the downgrade can leave a database in a fully downgraded state.