Do you also have the Koschei's Death Egg relic? It's required for the Deathless relic to work. If you wish, you may read more about that by visiting the two Wiki articles below:
TLDR: It's invisible, you'll get the normal xp but it will still look like it's taking xp away from you when calculating your floor score. You can look in the chatbox and it should say something like "your archaeology perk has stopped you from losing xp for this floor"
I'm sure you've fixed it now but to activate the relic, all you need to do is: go to the monolith with the appropriate chronotes in your inventory and simply activate it (or with a cape at a bank). Double check it's activated with the left 3 perks before continuing.
Now that you've got it activated, it's important to remember:
1. This relic only works in dungeoneering
2. This relic won't show deaths as 0.
Point 1 is fairly straightforward as that's the description of the perk.
But Point 2 is important to note. After some self-testing myself a few months back, I realised I was getting 20%+ penalties (hard mode don't judge me
). But I was still getting ~170k-180k xp.
This xp was coming from hard mode, party simulator (the invention thing), my dungeoneering level, bonus xp and the floor itself. I died many times for this one.
Re-trying with the exact same bonuses but taking a bit more care this time (reducing deaths from 10+ to 0). I got ~170k-180k xp.
One thing you should be able to see that's worth noting, is that in the chat box, there should be a little message saying about how you've not lost the xp it says you have due to your archaeology perk.
Just make sure it's equipped and explore those dungeons! Hope this helped.