I know, right? All these different definitions for regions. Humble, Steam, publishers, all can be different. Region locks are a real pain to deal with. Like the definition for NA games is USA + Canada excluding Mexico? Plus the definitions change, country names change, etc

If you wanted to recreate the giveaway, when selecting the region, use the predefined EU first. That will put a checkmark on every country in the list for that predefinition. Then type the country name in the search box beside it and the country will show in the list and you can uncheck it to remove it or check it to add it. Check the list of countries when the review giveaway comes up. You can go back and edit it again.

You could let the giveaway finish and ask the winner for country info. Check their steam profile. Friend them and ask. Leave a comment on a giveaway they made or their steam profile.