While playing TFT Ranked last night, I put a deathcap on Vlad to boost the damage of his ability. It worked with the item by itself on Vlad.
However after adding Guinsoo’s and Hand of Justice, the extra damage from Rabadon was removed.
I do not know which of those items caused it to cancel Rabadon (I added them both to Vlad at the same time) but I’m assuming it was the Hand of Justice since it also affects spell damage.
I’m not sure if this effect is something that can happen on other characters as well, I only had it happen with Vlad so far.
1. Get a Vlad
2. Build rabadon and place on Vlad
3. Notice the tooltip damage increase + effect working in game
4. Build Guinsoos + Hand of Justice on Vlad
5. Notice the tooltip no longer reflects extra Rabadon damage nor does it apply extra damage in fight