Edit whole OP:
So whenever I entered a battle in the Nemesis Survival Dungeon with full life, I lost a significant amount of life at the beginning of the battle. So eventually someone on Discord told me that this is an old known bug that team life runes do not work in Survival Dungeons. So if this is an old known bug - how come it hasn't been fixed yet??? We all know how critical extra life can be to survive specials and hits in general!!!
The way the bug seems to affect the workings of team life is that the monster wearing it actually gets the life increase but the remaining monsters on the team don't get it. However, if you have the Rune Guardian activated, the extra % of life does not count at all, not even for the wearer of the rune.
So @Carlos I would appreciate this to be fixed by the next round of Survival Dungeons!