Can you post an example of what you mean, i.e. a before and after?
I think from the description people are only seeing what selectors they can access via the set up of the section access. As far as i am aware that is how section access works when reducing the data.
An option could be checking peoples access on individual charts and objects and not reducing the data, but it can be a lot of work.
By using section access with strict exclusion a user could only see/access data to which he/she is associated to and I think this isn't possible with NULL even if you used a star is *; wildmatch - it will be applied only to real values.
This meant you need to fill NULL with real values, maybe like:
if(len(trim(YourField))=0, 'NULL / Missing Values', YourField)
More to NULL could you find here: Finding NULL.