It happens if you turn off god mode, and then they die, and you turn it back on, and you try to revive them.
The unlimited party health isn't actually god mode, they still take damage, what it does is it just sets your current HP to 1 million and tells the game to freeze it there but doesn't modify your max hp.
If you turn off god mode, you die, and then turn it on. The game probably thinks you're now at negative 1 million hp but it only recognizes your party member's maximum hp (say it's at like 600). Revive basically is a heal that gives you like 10% of your hp back. But if your current hp is negative 1 million and the game is trying to heal you for 60hp, then you're not going to revive.
This is also why Reaver spec/Iron Bull does no damage if you use the trainer's god mode. Because Reaver spec's damage is completely based on %hp and damage is multiplied by bonuses. Problem is, they didn't think you'd have 1000000/600 hp so the bonuses go negative and instead of getting 10% dmg for every 5% hp you are missing, you gained like 1000% hp so you lost 2000% dmg.
You misinterpreted me, the SPELL revive works fine with godmode (for me at least) it is manual revive that is stuffing up. So if I revive with the spell its all good but if I do it manually fks up.