It can have internal pressure leakage because of problems with the valve body. There is a ton of parts that have to be running within tight operating conditions for a transmission to operate properly. Lots can go wrong with any unit and it seems it happens more and more with a lot of OE's because of the complexity of today's transmissions over those of yester-year. The transmission you have is quite sophisticated and when running properly is beautiful like it's cousin 8-speed in the Rams.
They will put the VB in, do a relearn along with a fluid change. If that doesn't fix it be happy they are replacing the transmission because they aren't cheap units and you will get the benefits of any updates / corrections they might have figured out between the time yours was manufactured and the newer unit they put in.
They can try and engineer out all foreseen problems but until they are in service there is no telling what might become an issue and they have to revise it. And while your transmission design has been out for a couple of years already it's still really new in that application so it can be more problematic.
The 62TE in my Caravan has gone through numerous hardware updates and revisions and there are something like 28 different firmwares for it depending on the year and build revision just to give you an idea of how they keep tweaking things and correcting issues as time goes on, that were unforeseen in the original design. Mine shifts slightly different than my dad's from 2012 yet they are same transmission just with minor revisions/updates and software.
It sucks that you have to go through it but these things happen. Most of the units have been ok since early issues were sorted out. Sounds like you just got a problem unit more than a design flaw.
If these units were really that bad they would have dumped them quickly like they dumped the Jatco CVT's in certain models they were using them in because they were brutal for failures.
At least it happened now under warranty than down the road on your dime.
Hang in there... All will be well I'm sure, just sucks in the moment.