deformed baguettes with aluminum oven-roaster

Has anyone had their baguettes rise/spring in very unusual, deformed shapes when covered by an aluminum roasting pan?  I've been trying this method for a few weeks now, and invariably, my loaves do not expand uniformly along their lengths or along their widths (think of a hilly terrain).  They also end up somewhat flat (similar to what people have described when they oversteam....which has never happened to me, as I have a gas oven that doesn't ever retain enough steam).  However, I usually cover the baguettes only for ~6-7 minutes, so oversteaming seems unlikely.

Re: the nonuniform expansion in the oven, could this be due to a lack of proper heat circulation under the aluminum pan?

Thanks!

Steven