alpha matte in color page - strange fringing

Hi

I'm using a prores 4444 clip with alpha channel. The alpha works as expected in the timeline, cleanly masking out the correct part of the image. I want to add a mask to this and found that when adding one in the color page it overrides the track alpha / matte.

I created an external matte node and loaded the original clip as the matte. This works in that it allows me to combine the track matte with further masking, but there is a strange colour fringing at the pixels of the edge of the matte. Changing alpha mode to premultiplied made the colour fringing more severe, and purple rather than random pixel colours. At some point when changing the alpha mode back to straight, the fringing disappeared, ony to reappear after some other edit. This makes me think it's a bug rather than user error.

Do you have an idea either what I might be doing wrong, or if it is a bug how I might work around it for now?

Thanks

Here's an example of the fringing artefact

colour fringing.png (58.81 KiB) Viewed 830 times

and here when alpha is premultiplied

colour fringing premultiplied.png (57.7 KiB) Viewed 830 times

I noticed the same.
When I use a clip with alpha channel in the color page, and then turn on alpha output and add a mask, the strange fringing appears. Workaround is to use Fusion page for additional masking.

I'll do the same. Cheers