Sorry about this bug! It's a problem with Temporal Anti-Aliasing (TAA). Last year I investigated further into it and double-checked everything on the game's end was fine, for example that all objects are writing motion vectors. My guess is that it's a problem with skinned meshes (animated). There have been a bunch of problems with the built-in TAA in the past like the bug where transparency field of view got stuck with TAA (#1329 #558) that got fixed after engine updates. Last year I tried updating the post process version but ran into a different bug (#3235), so planning to revisit after the next major LTS update.