Thanks for the quick feedback.
I checked each of these four steps:
1) Yes – Smart Blend and Auto Allign are both checked
2) Viewer is set to RGB
3) I tried the different interpret footage settings – these affected the look of the key and changed the way the problem showed up (it became a flickering coloured ring around the edge of the footage), but didn’t solve the ‘grow’ problem.
4) Toggling between 16 and 32bit made no difference
I can’t post an example of the footage here, but i will send it to you at tech support.
Kate