When executing "ShowFlag.Rendering 0", it occur the memory leak because it can't run RHICmdList.ImmediateFlush().
The following code is a workaround. DEC_MEMORY_STAT_BY() is ImmediateFlush. Without this code, only INC_MEMORY_STAT_BY() is executed.
1. Launch standalone game with "-d3d12" argument from editor
2. Exec "ShowFlag.Rendering 0"
3. Exec "Stat MemoryPlatform"
Then, it can see increasing Used Physical memory.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-79676 in the post.