Thanks for the idea Grazie, still the same. It's frustrating! I've successfully masked an unwanted cassette player except for one frame, and I cannot find the same frame to mask in Pan/Crop. I'll try in an older version, maybe V15.

@PeterWright - Can you show me the actual timeline AND that P/c within the same ScreenGrab?

What if you just zoom out the view of the Pan/Crop canvas?

@PeterWright - Try unchecking Quantize to Frames and/or Snapping. If it is unchecked, try Checking! 🤔 As this is Interlace this might have an affect on fine Frame by Frame Keyframes. Just a thought.

Grazie, the screen grab shows Pan Crop and Preview, and you can see from the position of the sticks they're different frames.
Marco - no different if I zoom - it's still the incorrect frame.
I've got round it by rendering then re-masking two rogue frames.

Grazie, the screen grab shows Pan Crop and Preview, and you can see from the position of the sticks they're different frames.

@PeterWright - Yes, yes I realise. I want to see where you are on the TL and which Frame you’re on within P/C. We need some science to reference what we see against where you are within P/C.

Did you check the Toggling?

Geazie I'll attach the two screen-grab, but all it shows is that the cursor is somewhere along the timeline.
The central thing is that I'm changing the mask shape every frame for a while, and when I move a point of the mask, I should be able to see the result in the Preview window. If it is showing a different frame, it's no good.

I've toggled everything off and on again. I'm using Alt / arrows to move one frame at a time through the clip.

I can reproduce this issue, the pan/crop is sometimes not synchronized with preview. If I move the cursor in pan/crop window, synchronization fails, but if I now click "Sync toCursor" to off and than again on, the preview will be updated to the correct position..

Another try: If I use Vegas crop as Fx and not pan/crop, cursor is allways synched.

Musicvid - you've nailed it! This was a 1080p project, but that clip is 4k. They are both 25fps, but changing project properties to match 4k synchs things up again - thank you!

I thought that description rang a familiar bell!

Feel free to mark one of the replies as The Solution if you wish.