also check against length

src/config.ts

@@ -435,7 +435,7 @@ function migrateOldFormats(config: SBConfig) {
     }
 
     // populate invidiousInstances with new instances if 3p support is **DISABLED**
-    if (!config["supportInvidious"]) {
+    if (!config["supportInvidious"] && config["invidiousInstances"].length !== invidiousList.length) {
         config["invidiousInstances"] = invidiousList
     }
 }