I'm not this intepretation of the nature of Slavic populations make sense, as I understood their differentiation in 3 groups wasn't very old and formed as they migrated, so if they don't I'm not sure why one would expect South Slavic features to appear in Ukraine.Rather than simply in OTL Romania, might the South Slavs be present in what in our timeline is Ukraine, perhaps mixing with east Slavic dialects? This would lead to a clearer cultural separation from other eastern Slavs, which might lead to longer periods of being a separate nation.