Back in 2015 when WebStorage was originally replaced with Local Storage in Construct 2, it was noted that they aren't compatible:
Also note that since Local Storage uses a different storage mechanism, it will not be able to read any existing data saved with WebStorage.
WebStorage uses a different storage backend behind-the-scenes - one which is slower, has a very low capacity, and loses data types. Local Storage is faster, has much higher capabity, and preserves data types. You sure left it a long time to upgrade!