Glypheo Surtitles Not Showing Black on LG OLED TVs

I don't know if anyone has any experience with using Surtitles in a theatre space. We're using Glypheo and we're not getting a true black background, which gives a grey itglow on the screen, whereas a slide with black text for example is a true black. Any ideas on how to fix this? (we're using a Just add Power Av/Ip system)