I'm pretty sure that's not the case. After this long of time, the time decay should have pushed out the older posts by now (I'm seeing almost exclusively 4-5 day old posts on my feed). Using this as reference: https://join-lemmy.org/docs/contributors/07-ranking-algo.html#:~:text=Lemmy%20uses%20the%20same%20Rank,and%20the%20post%20published%20time.

If it really was an issue with Lemmy's algorithm, I think other instances would have that issue too, but visiting them, it's all updated correctly.