Tried to find something similar in the forum but came up empty, so I'll post it here.

When Deacon is not the active companion, he starts to gradually lose health and eventually dies. No clue what is causing the health loss, but the real interesting thing is that he also loses his essential character status, making him mortal.

I checked some things out in the console and I can confirm that when he is the active companion he is immortal (IsEssential 1.00) but becomes mortal when I switch to a different companion. The only other stat that seems out of place is that for some reason GetDisease returns a value of 1.00, and I didn't think that command was even used in Fallout.