As I commented in the issue tracker, this is actually not a bug: sorting is done by Solr, and trying to use anything else as a second scoring option will likely fail. The score is a float when Solr computes it, and the likelihood that two values in the result list will be identical is very small.
But I have the feeling you want to do something else: boost more recent documents perhaps?
Yeah I figured as much as far as a bug goes - it really working as expected although it's always going to have the weird effect when sorting with score and something else.
I don't think boosting will help - all that will do is change the score, right? It's really only that a customer noticed because they wanted a list of documents by score and then noticed the dates were random and asked if we could sort by date too. Maybe a caveat should be added to the documentation where it describes how to sort by multiple parameters.
I suppose there's no way to trick solr into storing the score as a int when indexing?
I just googled this and came up a bunch of people asking the same thing for solr going back until 2008 with no good asnwers.
You may as well close the bug and call it a feature. Although it would be nice to document it somewhere other than here.