I started randomly having this issue in one of my projects and it’s really annoying. Basically, when I play the game in an editor window, a media player plays a video in a UMG Widget, but when I then close that window, it doesn’t appear to actually stop or close that media player and it seems to keep running in the background and when I play in editor again, the media player plays a second time which causes the video to kinda skip between the two. This repeates every time I close and open a play in editor window and the video player will keep stacking and will eventually cause major editor lag when there’s like 5 iterations of the player playing. I can even see this when I open the media player asset as when I pause the video the, it seems to only pause the current iteration and all the previous ones keep going, despite being paused. This doesn’t appear to happen when I play it as a standalone game so I’m assuming it’s just an editor bug, but it’s still extremely annoying. It also appears to be localized to just that one project, as it doesn’t happen in another project that also has a media player playing a video in a UMG Widget with pretty much the same setup.
This was fixed in 4.18. Please update if you can.