In Chapter 71, Shadis says "I met Grisha... twenty years ago now?", meaning the year would be 830, but according to the pages on this wiki this happened 18 years ago and not 20. Meaning Zeke is 25 and not 27... was this correction made at some point in the manga or did someone think it was 18 years because of Ymir's Curse?
Shadis' tone suggests that he's uncertain of his own claim that it happened twenty years ago. Because of the 13-year curse, Grisha couldn't have been within the Walls for longer than that. Since he went with his choice to attack the royal family in the year 845, assuming he'd reached his final year, this means he would have arrived in the Walls in 832. 18 years is pretty close to 20, so Keith was just uncertain of the timespan. It could be that he hadn't seen Grisha for several years after finding him, which would've thrown off his memory of how long he'd known him.
That's definitely one of the things I considered... might have to look into the raw and the different translations though. Still, I think Ymir's Curse has some unexplained aspects... though considering Grisha wanted to show Eren the basement if he made it back, it's possible it was his last year... hmm.