Really interesting graphs!
Hmm I don’t think creatinine needs a spline - because when we fit a spline to that above the line stayed straight. Also this uptick is showing up non-linear on the curve for MyC already. Hmmmm, I wonder could this be a small group of people who have really bad Cr but relatively normal MyC ? You could try plotting a curve with a higher Cr value, 300, 400 etc and see what those curves look like. Or you could actually look in the data and see are there people who experience AMI despite normal MyC - and then look at their creatinine. Maybe there are a handful of people with truly awful renal function and for some reason normal MyC ?