Install another calendar app to fix this
I struggled to try to fix this with the help of this thread and others, so I thought I'd post the really simple solution I eventually found.
I simply installed another calendar app (in my case "Business Calendar Free") and set it up with my Google account. Somehow that resolved the syncing problem with the native Android Calendar app as well, so I now use that again as I don't like Business Calendar Free as much. But I've left it on there to facilitate the syncing for the native app.
Hope that helps!