Thanks GunSquid, I should've mentioned that I had tried that too. Tried again, no change. For the render I did "File > Export > Add to Render Queue", in case there's another way to do it. I rendered as AVI (but I'm guessing that doesn't matter since you said to delete the render anyway).
I'll try posting in the discord!
Update: solved! The issue was that I had the video file selected in the Projects tab (instead of the Composition).