In patch 13.1 Smite Vs Pets "broke", instead of dealing 500 + 10 max health true damage as it should have dealt, it would instead deal;
Smite level 1: 600
Smite level 2: 500 + 10% max health (Copy & Paste of Chilling smite which is why it kept the reduction)
Smite level 3: 1200 true damage
The reason why smite broke in patch 13.1 is very simple, whoever worked on the smite changes forgot to add the damage reduction to Smite level 1 and 3. This bug was reported but instead fixing the bug by adding the damage reduction to all 3 Smites they removed it with no information given as to why.
I assume someone thought the damage reduction on Smite level 2 was the bug instead of Smites level 1 and 3 missing it. As of patch 13.2 and onwards smite now deals full damage (600/900/1200) VS Pets/Minions instead of the reduced amount...
Proof:
Smite level 1: Dealing 600 true damage