Hi,
Thank you for the details, I was able to replicate the issue.
I stringly suspect that this is something on the server side, as the problem is not possible to replicate 100% of the time, it happens randomly sometimes on the same phrases, then not, then again, then not.. This is definitely not a programmatical error in the plugin itself, as it would be possible to replicate all of the time with the same parameters.
Do you happen to know if there is any object cache, or some sort of database cache active in your hosting package, on the server level? The requests are coming back with an “OK” response, so the code executes, runs and somehow there is nothing in it. I have seen previously this exact same issue with a PHP object cache enabled.