Certain tags within the Stripes Taglib do not allow you to work with Collections or Arrays. All tags that use InputTagSupport.getSingleOverrideValue() to obtain the override value for the field get returned the first value in the Collection/Array which is then passed to the Formatting framework to format. This prevents users from displaying a formatted list of the values, instead always displaying the first value in the collection, even when a CollectionFormatter is present.
What I think should happen is that the tags should use InputTagSupport.getOverrideValueOrValues() and pass the resulting Object off to the formatters. That way a CollectionFormatter could then be used to format the values accordingly, whether that be to return the first value in the Collection to maintain current behavior, or return a comma separated list of values so that data values are not lost. This also allows overriding to customise the output, which is currently not possible for Collections/Arrays.