Larian Studios
I am not sure if this is intended interaction but the "Rallied" buff that is given during the first goblin fight at the gate (when the fella blows the horn and rallies everyone) has a bad interaction with other temporary hit point spells. The rallied buff gave all my characters ""more hp" which I believe was 8 temporary hit points for 4 rounds. My warlock had Armor of Agathys on him, giving temp hp as well, so the rallied overrode the armor of ahathys and he lost it (which in 5e you can't have two sources of temp hp so this makes sense). However I then got hit for lots of damage, lost those 8 temporary hit points and tried to protect myself by casting Armor of Agathys again to give me a new 5 temp hp the frost damage explodes. However I still had the "rallied" buff so when I cast the Armor of Agathys spell it did nothing, no more temporary hit points (even though I was at 0 temporary hp). This did not seem like the intended effect.

I have not done any other testing with other forms of "target gets temporary hit points" but it may affect other cases temp hp as well.
I had the same issue with the Dark One's Gift. In my case, the HPs from rallied had been used up and then I killed a goblin. I would've expected the Gift to then take effect, but it didn't since the rallied buff still had several turns to go. Interesting thing was, after the rallied effect did wear off, that's when the Dark One's gift took effect.

The rallied effect (or any of the temporary HP buffs for that matter) should probably be broken once you lose all the temporary HPs.
I have warlock with false life and casting Armor of Agathys seems to do nothing in that case as well. I am guessing there are issues with any temporary hitpoint stacking.
© Larian Studios forums