Re any other plugin, I have no idea, Jeremy. Not even sure what to look for. This issue has arisen only since I updated JetPack to version 2.6 earlier today. No other plugin was updated today, although I realise that perhaps JetPack 2.6 may be clashing with an existing plugin. How would I troubleshoot that? (If it's disabling plugins one by one, etc, I'm not keen at all to do that unless it's absolutely the only way to find out.)
As for a function, I see nothing in any functions file or code that looks like anything to do with Open Graph. I've not added any function or hook to anything in WordPress for months. I'm not a programmer or coder, mind you, so maybe I don't know what to look for that isn't obvious.
As I mentioned earlier, I have not experienced this issue before at all, only since updating JetPack to version 2.6 earlier today.