Steps to Reproduce
This is reproduced in general by playing a movie rendered to 30 FPS on a TV with 59.94 FPS refresh rate or playing a movie rendered to 29.97 FPS and playing on a monitor with 60 FPS refresh rate.
I have added our reproduction case in vanilla UE 5.4.4. There are two UE projects. One suitable for 60 FPS screens and one for 59.94 FPS screens. Open one of the projects and play. Either you get no special hitches or you get a back-and-forth looking stuttering precisely every 16 second, that goes on for about a second. Which project gets the repro depends on the screen.