We upgraded a website to Magento 2.4.3 from 2.3.X
2.3.X haven't used Elastic Search, so we added it for 2.4.3
After the upgrade, we noticed a really weird issue: products in some categories (not all) are not displayed, they are completely empty. Also can't be found with a search.
We have checked every obvious reason why they can't be there - in stock for linked inventory source, the category defined, website defined. The product on its own product page is displayed and possible to buy.
Based on that something is probably wrong with Elastic Search, but we can't figure out what exactly.
Don't use any layered navigation extension, update other extensions in use when upgrading Magento.
If it makes a difference for ES we use AWS (with 7.10 engine selected) and we use MSI (default plus 2 sources, 1 source per website).