The wording is (as usual) poor. It really seems like 'beginning in 2025' would modify "following the date the Milestone is achieved".
So this would only apply going forward as the milestones are earned? But, alas, the complex, run on sentence structure. Further, they knew that people were waiting for 1Jan. Could they not give an example with a 2024 milestone earning?
I guess we will know tomorrow when someone takes one for the team and chooses it to overlap an existing one.