It’s just a change in wording. Right now talents do a poor job of conveying whether a bonus is additive or multiplicative (I really wish WoW could take a page from modern ARPG books and just a bracketed + or x to indicate this).
Currently Echo Chamber is an additive 10% bonus, meaning it’s 25% + 10% = 35%. It is being changed to a 40% multiplicative bonus - in other words, it will be 25% x 1.4 = 35%. It’s still meant to be the same amount of damage.
Whether this is purely a change in talent wording or an actual change in calculations under the hood I do not know, but it’s possible this change was done to more easily code the bonuses to new Ascendance.
The main reason you stack mastery was Ascendance but this is ridiculus anyway.
I’m absolutely mad. First of all this garbage rework and on top of it because of lazy M+ players they nerf Poison Cleansing Totem to 2 min. CD! Just because average M+ noob cannot take dispel on DPS spec. They cried that R Shaman is op but he can use the totem and deal with mechanic himself while you could literally do it with any healer + 1 dps being specced into dispel. Literally every dps spec could deal with it. Hunter could Feign Death the mechanic, Rogue could help with Crimson Vial and in worst case CoS, Warrior could use impending Victory but no better nerf class in PvP against Assassination because people cannot sacrifice 1 talent for utility.
As the OP said, the Ascendance change is the big mover in Mastery value. It’s worth noting that Stormbringer gets a lot more value out of mastery than Farseer too, and the meta is expected to change from Farseer to Stormbringer. If you are staying Farseer then you are still going to want a lot of haste.