Grid pattern appearing in grain when exporting in jpg

Exporting film scans in jpg (large) creates a strange grid pattern within the grain structure of the film. Exporting with jpg (small) and original + settings both work as they should. Any ideas? Same issue happens on both mobile and desktop. Film is pushed hp5+ so the grain is very pronounced. I have seen suggestions of using noise reduction to combat this but I would rather not do that. Also, the possibility of lens corrections being the culprit can be ruled out here as there are no lens corrections active. The image has not received any editing besides a straighten.