So, the 4th Atomic heart DLC will also follow the canon ending timeline.
This means only the first DLC isn't canon, and honetly, it seems like a waste fo a DLC, but still, it's a pretty unique thing for devs to make DLC for their non-canon ending.

Mundfish teased the last DLC for Atomic Heart, DLC 4, it'll probably take place after the Trapped in Limbo DLC, and therefore take place in the canon timeline, being set directly before Atomic heart 2.