Opacity issue on .png for green screen in iMovie

Hi there!

I've been using the green/blue screen option in iMovie for a while to place logos and titles over videos. I made a png in Photoshop and then place the png into the timeline, change the settings to green/blue screen, and it usually works fine. However, in the past few weeks, it has not kept the image solid and instead makes the logo etc. look slightly transparent.

For example, here's what my png looks like before I change to green/blue screen:

When I put it on green/blue screen, the logo is no longer fully grey and shows some of the video behind it:

Changing the "softness" level does not affect it.

Is there a setting that could be causing this? Any help would be appreciated!