This is a regression introduced by the particle system manager. It does not account for the owning actor's time dilation when ticking the particle system components.
The particle system will simulate as if the time dilation were 1.0.
NOTE: If you disable the particle system manager with the command "fx.PSCMan.Enable 0" it will simulate at the owning actor's time dilation.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-88322 in the post.