Car stalls going into gear

I have a brand new battery and it's supposed to handle cold weather -30.
I've got a jump starter because I need jumps so often.
Now when I put it in gear it stalls and totally kills my battery and I need to jump it again.
I even tried to put it in gear with the jump starter still connected to the battery. It almost shut off again but I put it back in Park and revved the engine again.
I could hear it start to shut off when I put it in gear.
And my alternator has been tested by a licensed mechanic and it's fine.
In the past and I was able to put it in gear it would stall while driving too.