We made some changes to how movies are buffered and rendered in 2025.2.0 ( Release PsychoPy 2025.2.0 · psychopy/psychopy · GitHub ) so I think this is a result of those. The key difference is that movie frames are now buffered asynchronously, which allows for easier seeking to timestamps and is generally better for stability and speed, but it looks like there’s sometimes a frame at the beginning which is allocated for before any data has arrived (so displays as black), hence what you’re seeing.
This is fixable I believe, and @mdc is already looking into it. I’ll report back here when we have a fix for this 