I couldn't find another note than the one Duke already mentioned.
Did you read that?
"One workaround for this problem is to define the entire expression as a
macro earlier in your job definition, then invoke this macro in the
Expression Builder window."
Not that this is pretty but: You could assign the whole expression to a macro var in the pre-processing part of your job and then just call this macro var in the expression builder window (I think that's under advanced where you can write your own expression - click, click...)
I investigated a bit more:
DI doesn't let you save too long lines (>254) as such lines would get truncated in SAS batch processing.
Linus got it once more right: Just add line breaks and everything will work fine.