[mention removed] - I’d like to make it clear - this is not an issue of the Time Dilation value being a frame behind.
The problem here is that the “InPlayRate” parameter that Unreal passes into IMovieSceneCustomClockSource::OnRequestCurrentTime() is being set back to 1.f every other frame, during an active time dilation track in the level sequencer. This behavior does not occur when testing the level sequence in the editor, which seems like a bug.
I’ve finally got it working by locating the SlomoTrack Section and querying the Time Dilation keyframes from it manually. This required some code to only modify the DeltaSeconds on my CustomClock when we are in an Editor World (since the DeltaTime is automatically scaled by Time Dilation when playing the game as you said above).