I kept seeing the following error in system.log
“main.ERROR: mysql2 search engine doesn’t exists. Falling back to mysql  ”
The fix for me was to go to Stores > Configuration > Catalog > Catalog under the section “Catalog Search” you’ve probably got MySQL selected for “Search Engine” and you’ve probably got “use system value” unticked. For me ticking “use system value” made the error go away.
I did see another fix for this which was to run the following sql query “UPDATE core_config_data set value = ‘mysql’ where path=’catalog/search/engine’;”