Question about Additive Composite

I'm trying to use additive composite to layer one clip over another (it's an old school style title card with white text on a black background).

My understanding is that by using add to apply this layer, anything underneath should only get brighter. What is happening instead is that areas that are white are getting lighter, but areas that are black are getting darker.

What am I doing wrong? Thanks!

Are you doing it in Fusion page or Edit or Color? And what kind of color management system are you using?

Add blend mode shouldn’t darken anything, there are only two cases where this could happen: a) there are negative values (either in original source or produced due to color transforms) or b) blend merging is buggy.

Ok thanks. It's just on the timeline. I'm guessing it's the negative value issue as I've used a curve to bring the blacks down!