This article describes some workarounds/fixes to temporal reprojection issues encountered when using volumetric fog with UE5’s Lumen lighting system, including artifacts that can appear during object or camera animation, particularly in shaded areas.
https://dev.epicgames.com/community/learning/tutorials/6dJK/unreal-engine-lumen-volumetric-fog-temporal-reprojection-issue-workaround-icvr-lightfall