It is wierd. This is just a guess - Could it be that the recovery model may be changing which is causing the log to flush over to the data files and drop down to 1GB itself?

Another thing is that although the database is restored on a SQL 2008 R2 environment, it appears that the database is running under the SQL 2005 compatibility mode (90). Not sure if backup compression on a database with compatibility level 90 has something to do with it.

I don't have a SQL instance right now to test it on, but will give it a shot as soon as I have one.