Magento 2.35p2 EE with AWS ElasticSearch 6.8 – wrong products being returned

We recently upgraded from Magento EE 2.1.18 with ES 2.3 to Magento 2.3.5p2 with ES 6.8 on AWS.

The previous version of the system returns correct results when a user specifies a search keyword.

In the 2.3.5p2 version we are getting extraneous results – products that do not contain the search word are being displayed on the results page.

In examining the search logs in ES, the search query that Magento is generating looks correct.

There are no references to the incorrect products, yet on the results page the incorrect products are displayed.

How does Magento use the search results from ES to identify the products in the magento database to display?

Thank you.