The graphical changes were apparently brought about prepping for Path of Exile 2 (read our hands-on here) as the team switches to more PBR (physically based rendering). If you’re unfamiliar with bloom, it’s basically a simulation of light diffracting through the virtual camera.
However, this effect can be annoying if used incorrectly. To that end, Grinding Gear Games upgraded the bloom effect for Harvest,
“With the release of Harvest, we upgraded to a new Bloom system that looks a lot better while having the same base performance cost as the old system. Because this system handles Bloom properly, the "Fake Bloom Particles" are no longer needed and have been painstakingly removed from as many effects as we could find throughout the game. This was a long process, but improved performance because effects don't need to have so many particles now (as the Bloom part of it is done at a low performance cost by the Bloom shader).”
However, players don’t like the new bloom, and so the team has provided a few open options on where to go. For one, they could fix the current bloom. They could also allow players to totally disable the effect by restoring the previous fake bloom particles. However, there isn’t an easy solution,
“We understand that for the players who find Bloom uncomfortable, this is probably quite frustrating. We're sorry that there isn't an easier path to resolving it, and that fixing all the cases is taking time. We expected to get most of the bad cases fixed before the release of Harvest, but effects of lockdown made this more difficult than expected.”