I did not see your ISNULL statement at the end.
If you use an ISNULL statement you have to make it the first condition in the entire statment.
As for my other suggestion you might not have this option. I am using CR XI. Other users have not found the option I am referring to but ... inside the formula editor there is an drop down list option to choose how to handle null values as either null or as a default value.
if your data is NULL instead of an empty string you have to decide how you want to deal with it. If you do state how to handle a NULL or use the option I mentioned above a formula will stop evaluating when it hits a NULL and return nothing.
I also have never had luck with using "X to Y" on an alpha.
Maybe try left(uppercase(field),1) in ['A','B',...]
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