The Hydra boss doesn't drop any scales from its treasure bag. There also seems to be no expert mode item from the bag (I am on master mode). Also, there is no bestiary entry for the hydra itself, only its heads. I am on tModLoader v2023.6.25.31 (Terraria 1.4.4.9), and I am on Qwerty Mod v0.9.0

UPDATE: After further testing, it seems that the hydra scales CAN drop from the treasure bag. HOWEVER, it is only a surprisingly low chance to be dropped (at least less than 20% chance), whereas I expected it to be a 100% chance to drop (Like shadow scales/tissue samples are always dropped). I have been unable to obtain any expert drop (I believe from previous versions they are wings)