I regularly see products disappear from Magento storefront categories with message: "We can't find products matching the selection".
After manually running indexer:reindex, then cache:flush products re-appear -- only to disappear hours later AGAIN.
I have a clean Magento 2.3 and a 2.4.2 install. Products are added manually. Stocks are correct. I don't see any issue with ElasticSearch (don't think ElasticSearch has anything to do with it). CRON runs every minute. Products are associated with the right site and respective categories. I'm running PLESK on Ubuntu. CRON is running via PHP 7.3 for Mage 2.3 and PHP 7.4 for Mage 2.4.2
Any help would be greatly appreciated.