We've seem that a few times too. We logged a case with support (...) but no solution so we ended up "cloning" the campaign in SFDC and repopulating with all the campaign members - It worked perfectly (go figure...). I also learned that if you updating the campaign membership status in SFDC it will "force" the sync with Marketo and you may see the members added in Marketo as new campaign members (yes, it will create activity history in Marketo too). Lastly, match the count in both platforms to avoid surprises...
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