Hi,
The plugin requests the result titles from WordPress (via get_the_title) – and those should be rendered already, meaning all the hooks, filters, modifiacations and anything else should be done on them by the time of the request.
We are also using Yoast on one of our test servers, but the titles are rendered just fine, so I am not sure what exactly is going on here.
Can you please check if the custom ajax handler option is disabled. When enabled, I can imagine there is a delay on the yoast plugin loading.