diff --git a/client/src/pages/atlas/useAtlas.ts b/client/src/pages/atlas/useAtlas.ts
index 6e5f2422..d8dee913 100644
--- a/client/src/pages/atlas/useAtlas.ts
+++ b/client/src/pages/atlas/useAtlas.ts
@@ -141,7 +141,10 @@ export function useAtlas() {
         for (const f of geo.features) {
           const a2 = f.properties?.ISO_A2
           const a3 = f.properties?.ADM0_A3 || f.properties?.ISO_A3
-          if (a2 && a3 && a2 !== '-99' && a3 !== '-99' && !A2_TO_A3[a2]) {
+          // Only real 2-letter ISO codes: natural-earth uses subdivision-style
+          // values like "CN-TW" for Taiwan, which would otherwise overwrite the
+          // legitimate TWN->TW reverse mapping and break the country (#1049).
+          if (a2 && a3 && a2.length === 2 && a2 !== '-99' && a3 !== '-99' && !A2_TO_A3[a2]) {
             A2_TO_A3[a2] = a3
           }
         }