Do you mean the referendum held in the eastern half of Donbas under control of the Russian backed militias?
That’s the one.
Ukraine’s Oblast is not like American statehood; their country is not devolved so any referendum on independence or presidential election to change government has to include all eligible citizens in all Oblasts.
Because the Donbas referendum and the Crimean one both DELIBERATELY excluded over 80% of eligible voters they are legally nonbinding.
Some say the same rule disqualified Zelenskyy because the separatist regions couldn’t participate. That is plausible, but sauce for the goose is sauce for the gander.
If Zelenskyy is not legitimate, constitutionally, then neither is Russian supported independence or annexation.