Exported Images Appearing as Negatives

I am a graphic design professor teaching 1st year students. For their first project, they are learning to manipulate color modes in InDesign, and are using placed grayscale TIFFs to create monotone and fake duotones. For several of my students, when they are exporting these images as PDFs to print, the export is resulting in inversed color images - almost as if InDesign is creating a "negative" version of their imagery. I have walked through the students several times, and the appearance of these "negative" images seems to be occuring almost randomly - I have not been able to find any correlation between an export setting or color mode and the production of these inverse images. We are all using CC 2021. Any clue as to why this would be happening? I am at a loss.