Autosuggest in 7.5.1

We have a working integration running a 5.6 ElasticSearch deployment on elastic.co connected to a site using the elasticsearch-php Branch 5.0 as suggested in the Version Matrix
Everything works fine here, including our autosuggest endpoint which returns results correctly as shown in this example response.

We are attempting to upgrade to ElasticSearch 7 so created a new deployment in elastic.co with version 7.5.1 and upgraded elasticsearch-php to Branch 7.4. The problem we are having is that the autosuggest endpoint stopped working.

For the same request that before returned the results above, it now always returns no results as shown here

We see nothing that points to what the problem can be in elastic.co logs, nor in our application logs.

Something strange is that sometimes the autosuggest endpoint returns 504 gateway timeout, but with no further indication or logs to help us point where the problem can be.

Any ideas, recommendations on how to troubleshoot this issue would be super appreciated. Can share example requests and further details.