diff --git a/src/plugin.ts b/src/plugin.ts
index 54f3ea5..56efbb9 100644
--- a/src/plugin.ts
+++ b/src/plugin.ts
@@ -499,7 +499,14 @@ export function legacySubtitleMigration(existingMeta: Record<string, unknown> |
     const langs = legacyCsvMembers(existingMeta, 'subtitleLanguages');
     if (langs) {
         deletes.push('subtitleLanguages');
-        for (const l of langs.map(toLang3)) if (l !== 'und') sets[`subtitleLanguages/${l}`] = 'true';
+        for (const l of langs.map(toLang3)) {
+            if (l === 'und') continue;
+            sets[`subtitleLanguages/${l}`] = 'true';
+            // …and the union (§9 rule #7). The live write path below already
+            // does this; the migration path did not, so records that only ever
+            // went through it were left half-written.
+            sets[`languages/${l}`] = 'true';
+        }
     }
 
     const subs = legacyCsvMembers(existingMeta, 'subtitles');
diff --git a/test/extractor.test.ts b/test/extractor.test.ts
index d8cf9cf..0bb4d56 100644
--- a/test/extractor.test.ts
+++ b/test/extractor.test.ts
@@ -310,7 +310,13 @@ describe('legacySubtitleMigration', () => {
             subtitles: 'cidS',
         })).toEqual({
             deletes: ['extractedSubtitles', 'subtitleLanguages', 'subtitles'],
-            sets: { 'subtitleLanguages/eng': 'true', 'subtitleLanguages/fre': 'true', 'subtitles/und/cidS': 'true' },
+            sets: {
+                'subtitleLanguages/eng': 'true',
+                'languages/eng': 'true',
+                'subtitleLanguages/fre': 'true',
+                'languages/fre': 'true',
+                'subtitles/und/cidS': 'true',
+            },
         });
     });