Hey, I got the same issue and found the cause for me at least.
In the previous versions of the engine, it would by default upscale post processing and lighting, where as now it instead renders them at (at least) full resolution. It appears a lot of scaling functionality has been redone and despite inheriting the old settings, those settings don’t affect things like PIE the same way any more.
Note that some engine scalability settings may also have changed, E.G as of 5.2 AA at Epic is 200% render scale rather than 100%.
5.1 vs 5.3:
Sadly, I have yet to find a specific setting to replicate the old behaviour aside from just setting the viewport scale, but that affects render scale/resolution as a whole and not just GI and PP, and is separate from the packaged game.
(Frankly, there’s too many settings you can set from too many different places that affect the various systems in different conflicting ways now, and what group of settings is intended for which situation is very unclear and a mess, viewport settings need to be clearly seperated from game settings, then you have editor vs engine vs project and console commands on top of that, as well as additional duplicates depending on what menu you access.)
It’s a shame because while it’s technically higher fidelity, there’s really no noticeable difference at least with my game’s artstyle, but a much, much worse framerate.