Multiply blend mode not rendering correctly (2016 project in Premiere 2025)

That gray bar on the right side is a white matte. The original footage had to be reframed, so the matte is there to fill the space. Both the matte and the background in the clip are pure white (#FFFFFF)

There is a slight gradient in a layer over the entire frame. It's at 25%, set to MULTIPLY blend mode. As you can see, even though the bg and the matte are the same colour, the overlay isn't being applied uniformly across the frame.

This is a project from 2016 (maybe 2018?) that I've been tasked with updating. There are 900+ shots like this, so ideally the fix is just a setting that I need to change somewhere.

I've already tried nesting the matte and the footage, but no luck. The footage is REC 601, so I tried changing the colour space of the footage, the matte, and the overlay, but again it didn't work. 

Any help is appreciated. Thanks