[Official changes]
W Infernal Chains cooldown:
Rank 1: 20s → 18s (2s faster)
Rank 2: 18s → 16.5s (1.5s faster)
Rank 3: 16s → 15s (1s faster)
Rank 4: 14s → 13.5s (0.5s faster)
Rank 5: 12s → 12s (unchanged)
E Umbral Dash health heal ratio per 100 bonus health: 1.1% → 1.3%.
[What changes in play]
At low W ranks, the window between a missed chain and the next attempt is slightly shorter. At maximum W rank, the listed cooldown is exactly the same as before. The E line is not a flat increase to every heal: it changes the ratio applied per 100 bonus health. Read it in the context of your actual item and rune build, rather than assuming the same gain in every game.
[How to check]
In the practice tool, compare W's displayed cooldown at ranks one and five, then inspect the E tooltip before and after gaining bonus health. Ability haste, the enemy's mobility and your spacing after the chain lands still determine whether a second cast is useful. Instead of engaging automatically because of a two-second reduction, look for the moment after an opponent has used a key escape. Riot describes this as compensation after an earlier item nerf weakened Aatrox.
Source: Riot Games, League of Legends Patch 26.19 Notes
https://www.leagueoflegends.com/en-us/news/game-updates/league-of-legends-patch-26-19-notes/