I am using VSE to layer a scene (alpha over) over a video created using a different app, rendering the result as a jpg sequence.
This was all working well, until I made some changes and, rather than rendering from the start, decided it would be more efficient to re-render only the modified frames.
When I changed the start frame in the render tab, rendering started from the wrong frame of the scene, although the background video was still being rendered correctly. This was the case regardless of whether the sync mode was set to AV, none or frame-dropping. I also noticed that the cursor jumped around during rendering.
After some experimenting, I realised that the VSE was offsetting the scene by the same amount as the starting frame; e.g. if I start rendering at frame 100, it would begin rendering frame 200 from the scene over frame 100 of the background video.
I thought I must have inadvertently changed some obscure setting, so started a fresh blend and found the same behaviour, even if there is no background video; as soon as you add a scene to the VSE, and begin rendering from a frame other than 0, Blender will start rendering the scene at twice the start frame set in the render tab.
Now that I know this, it's only a minor inconvenience, solved by offsetting the scene in VSE by the same amount as the start frame. However, I'm curious - is there a reason for this odd behaviour, or merely a bug?