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Workaround fwiw: I had this problem when going from one video file scene to another. I was able to avoid the flash by adding a full black screen jpg source under the first video. I also changed the first video from internal player to VLC. The second one is the internal player. I don't even remember what inspired this idea, or which one worked, but I hope something here helps.

I seem to be having this issue again with 26.1.1

I can confirm this is also happening to me on 26.1.1.

Noting that this continues to be a problem in OBS 27.2.1 on Mac. This old forum post does a really nice job of visualizing the issue: https://obsproject.com/forum/threads/media-source-rendering-the-last-frame-at-the-beginning.110180/

I don't even understand my own workaround above.
The one I use now is to put a very short black screen video file as the top item in the scene. When I switch to the scene, it plays the black frames on top and then that video stops, letting the thing underneath show through.

Problem persists on 28.0.1

Problem persists on 29.1.3
It's really annoying.

Problem still persists in 2025. I have many projects halted due to this, any help would be appreciated. How do we escalate?

If I were still dealing with this, here's what I'd try as workaround now:
use the Advanced Scene Switcher extension plugin
Things I'd try...
Macro 1:
On scene change, reset all media players to no media (empty)

Macro 2:
On scene change, if the new scene is the one that should be playing media, then set the media source and immediately play it.

If that doesn't work, then you could also put a cover image, or even a text box with a solid background, over the video and use Advanced Scene Switcher to turn it visible in macro 1, and hide it after 100 milliseconds or something in macro 2.

And a third idea:
Would it hide the trouble frame if you set up a 500ms fade-in as the "Show" transition for the video control? Then instead of having a cover-up object, you could tell Macro 2 to make the video itself appear, and perhaps the fade-in would hide the problem.

Oh here's a variation:
Do #3 using a very short blank solid video as a stinger. The advantage here is that it can pop in, rather than fade in, and the audio won't be affected. I think #3 might cause the audio to fade in.

I haven't had to deal with this issue in a while but I thought these might be good ideas. :)

I'm still using 29.1.3 but now the problem seems to have resolved itself. :S Maybe the problem was with Windows, and after an update it started working normally. I don't know.

On the stinger idea, I don't know if that would work, since a stinger is also just a cut with an overlay.
If that doesn't work, maybe try a custom Luma Wipe (there's a way to add custom wipe PNGs, see here), but use a blank white image and set softness to 0. That will look like a cut with a small delay, but it should trigger the playout at the very beginning.

I'm on macOS with OBS 31.0.1 and the problem remains.

I just tried some of @therentabrain's proposed workarounds (thank you for typing them out). Using Advanced Scene Switcher and setting it to blank out the file path and hide the scene item visibility, then restoring the scene item visibility and source settings does help but it's not a great solution for me. It removes the most obvious part of the painful flicker where you see the last frame of the media, but there's still a flicker as the video is now loaded in after a slightly longer delay. And of course there's now a lot of setup required in advanced scene switcher.

Thinking through this problem, I think what's really desired here is that when returning to a scene with a media source set to restart after played (but not set to loop), OBS should have the first frame of the media source preloaded and ready to go, rather than the last frame or no frames.

I've had a bash at resolving this and it seems to be working in my debug build. I have absolutely no idea on the ramifications of what I've done however, so anyone who could review would be greatly appreciated: Poodmund@4986749.
https://www.youtube.com/watch?v=gjyV74gqOFI