I had similar cracks show up a few weeks after decorating, and it turned out the decorator hadn’t prepped properly, just skimmed over old filler and paint. I ended up going with
https://craftsmanssealpainting.ca/ for a second opinion, and they explained how important proper joint taping and flexible caulking is to stop reappearing lines. Prepping is half the battle, especially with older plaster or where surfaces move a bit.