It was only ever retroactive in the sense that you didn't need to reach Fabled again after hitting that step of the quest if you had reached Fabled earlier that season. Luna's, Mountaintop, and Recluse are the same way. It doesn't mean you could have gotten Fabled in a past season and have that count in a later season.
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Faji Fox
A fox name Faji! - alt
Wish that wasn’t the case. Thought it would be like the 5 valor resets for the broadsword, even carrying over seasons when that step wasn’t up yet.... -
Can confirm that the redrix 5 resets also had to be in the same season when you got to that step. Source: I missed my redrix the first season it was out by 1 crucible bounty prior to season reset. After season reset, i did my bounty and then it had me at 0/5 resets. It has to occur IN the season when you reach that step (but can be done earlier in the season before getting to that step).