Open captions rendering with opaque background

I'm working in a team of 3 and we're having an issue when exporting videos that have open captions in them.

We've set all the captions to have black backgrounds that are 90% opaque, this is because we have to be consistent with the style established by another team that made captions previously in the titler.

Now the problem is that sometimes (mostly) when my colleague renders out a video with these captions, they come out looking almost completely opaque.

They look fine in PP during editing and they actually come out fine when I render them.

You can see below screenshots of what it looks like vs what it should look like:

The main difference in our setups is that he's using a Mac and I'm on Windows.

Otherwise, are there any settings that could be causing this? I'm led to believe that it's just a bug and there's nothing that can be done, since there have been other inexplicable issues like flickering frames that are fixed by just rendering on my machine instead. My other colleague is also on a mac