Enable Null Support issue in CF2018

I'm setting up CF 2018 (with Update 5) and am having an issue - it seems that "Enable Null Support" option in CF Admin does nothing. 

I have some API results that can return null values - eg; json {"FirstName":"Fred","LastName":null, etc} and when I cfdump the struct it has "undefined" as the value for LastName (after doing a deserializejson()). In ColdFusion 9 (which I'm porting over to 2018), the value for LastName shows as "null". My issue is that in CF2018 when I try and access this variable it throws an error "Variable LastName is undefined".

It doesn't seem to make a difference if you check the box for "Enable Null Support" or not in CF Admin. I've tried selecting it, un-selecting it, even restarting CF after each update.

Is this a bug? I'm using CF2018 with Update 5 (version 2018.0.05.315699). Any ideas?