Another possible issue is that one document (the one that works) has a stylesheet associated with it, while the other doesn't.
When that happens, Editor uses dflt.fos to display the format.
In 5.3 and before, this wasn't an issue, and both would behave the same. However, in 5.4 and later, the dflt.fos now has the quadding set to asis, which means it's trying to insert an carriage return when you hit enter.