Problem exporting green screen video with Ultra Key effect

Hi, I'm working on a video with a lot of green screen segments. I'm using the Ultra Key effect, and in Premiere's program monitor the results look fine:

But once I export, the resulting file shows a completely unrecognizable result:

At first I though I might be missing some important detail in the export settings, but after some tests I can confirm that the exact same problem occurss both when exporting an h.264 and a ProRes 422 - so probably there is something else going on.

The effects look like this:

Nothing out of the ordinary in the sequence:

My MacBook:

Could it simply be a processing power problem? Or can anyone think of other troubleshooting measure I could take?

Thank you!