I've had this puzzle on my phone for years. Every few months I'd glance at it and try and solve it. Last night, as I was eating dinner, I finally figured out the answer!

https://sh.itjust.works/post/55641581

For everyone saying it’s the difference between the two numbers, it’s not it. Because the last one should be 8 to follow the rule
Oh shit, I thought it was OP just being a dummy. Turns out I’m the dummy