Guinsoo’s Rageblade has been a bit of an issue for the past couple of patches with how it interacted with Jhin, but now it’s been fixed in patch 8.9.
Rageblade had a bit of a wonky way of working with Jhin’s passive, giving him upwards of 3,000 attack damage if he built it later in the game. This issue even came out in LCK, where players weren’t allowed to buy Rageblade on Jhin. This was never really a bug — it was working as intended, but it definitely shouldn’t have been doing all that.
Jhin’s passive, Whisper, was essentially going into a near-infinite loop with Guinsoo’s Rageblade. Whisper would give Jhin some bonus AD based on Rageblade. From there, Rageblade would recalculate based on that new amount of AD and then Whisper would do it again and then Rageblade would do it again and then ... you get the point.
This was fixed by having multipliers look at the same baseline instead of interacting with each other, according to information Riot gave to The Rift Herald. This means that Riot will eventually have to rebuild the item’s interaction with every champion, but the effect the item was having on Jhin were bad enough to warrant this fix.
Rageblade has been fixed, though Jhin’s passive is still calculating things in a bit of a crazed loop. This fix will have to come through in a future patch.