I had a similar experience with my Atlas a few years back. I replaced my transfer case oil when I first received my van (used) with RedLine MT-80. At the time I was using a lot of Redline Oils in "other vehicles" so it's what I used. Back then I was mostly in So Cal and every once in a while it would drip a little out the breather. Didn't think much of it. Then I moved to Colorado where 85 MPH is pretty common on the interstate, for longer distances, and up bigger mountians. I started to see more dripping out the breather so I thought maybe I should change to an Atlas Approved oil. Got in a hurry and put Mobile 1 in and headed over the rockies to Grand Junction. I pulled off on Vail Summit to find the entire underside of the van covered in oil with little in the TCase. Limped home and did the same routine of blowing air down the vent tube. Nothing, all clear. So I did some research to find foaming issues pretty common to Mobil 1, Amzoil with little to none, and Redline somewhere in the middle. I have since switched to the Amzoil (should have just used it from the beginning as it's the real spect oil) and have had no problems.
A couple of thoughts though.
My breather terminates up on the bell housing of the transmission so a little higher than the TC.
The gears in the Atlas have a pumping action to lubricate everything so I can see how an oils anti foaming property could be important.
It's easy to overfill the Atlas more than the two quarts specified. Especially if it is drained by the drain plug. Because of the position of the drain plug a few ounces will remain. I wouldn't trust the zip tie on mine to mark where full is. This last time I removed the base plate to confirm it was empty and I just wanted to look inside. BUT, if you do remove the plate don't get mid-evil tightening the allen bolts. They are only threaded into the aluminum case. Or you find yourself becoming an expert in the TimeSert process.
Use the Amzoil they specify.
Hope this helps