There are two basic features of list objects:
- they can be flattened, i.e. nested lists are reduced to a single list. this seems to depend on the optimization setting
- multiple values can be removed (this is controlled by a global setting).
The different behaviour for symbolic and numeric optimization is a bug IMHO.
In order to fix this bug and allow for flexible use of the list object, I recommend to not perform any of the two operations (flattening, compressing) by default, but provide appropriate explicit functions, say flatten() and setify()