I'm making a video for my Youtube, don't ask why I use blender for it, but for some reason the preview is completely different from the render output. In my video, I used one long clip and cut it into many pieces while in edit, but in the render, it seems blender just used the original clip playing, without the cuts. This is weird because all the other effects I have work as I wanted, but the this clip here doesn't seem to match the render at all. Here, the render (frame 2189) is seen next to the preview of the same frame. Why is it different? (Ill also note that switching to older versions like 2.7 fix this issue.)
I've just had this issue in Blender 3.0. It was due to the screen recording footage I had being a variable frame rate.
You can check if yours has variable frame rate (and amend it to be constant) using
Checking if you have variable frame rate: https://www.addictivetips.com/windows-tips/check-video-variable-framerate-on-windows-10/
Did you check in the Properties tab, on the icon that looks like a printer, at the Post Processing section if either the Compositing or Sequencer box is checked?
The Compositing outputs the Compositor Editor or the 3D Viewport camera while the Sequencer outputs the VSE/Video Sequencer.