The previous version of MultiStringValuesProperty was MultiStringValuesIndexingField. It wasn't a indexing field on string values, but indexing values from multiple source as string. It used ContentSearchHelper.getAttributeValues(...) to get and transform attribute values (take a look to ContentSearchHelper._transformSingleValue(...)).

So now, some properties which are old indexing fields with class MultiStringValuesIndexingField does not work anymore.

One of the consequence is that the content which this kind of properties is not indexable anymore.

Note that the initial field was designed to managed mixed types, I do not think that a solution with typed properties is a good one.