A saw one at a contractor recently. It had these disgusting "bubbles" in the central flexible area. Like a screen protector that has bubbles, but it was part of the screen; there was no screen protector on. Like this: https://r1.community.samsung.com/t5/galaxy-z/samsung-z-fold3...
I'm pretty confident that it's the screen protector peeling off around the crease area. The Z Flip / Fold comes with pre-installed screen protectors, and they kinda sucks - always peeling off after a few months. After replacing it once just to see it bubbling again, I decided to take it off entirely. I haven't seen any problems except that the screen now attracts scratches like crazy.
