That's the one that germinated first..so it's about 5/6 days ahead and it's the one that started to look like this first. The other one in front of it started to show slight yellowing as well which was the next in line to germ, and the other two seem to be happily green.
I'll give it a few days to see whether the reduced feed will kick things back in life.
I've noticed that the buds haven't changed/grown a lot since I started to have these issues. They've stopped stretching for sure but at week 6 flower I would expect this.