Recently we experienced this very issue too. Solution in our case was making sure the inventory_source_item MySQL table does not contain any defunct SKU's. To lookup corrupted records in said table, try next MySQL query:
SELECT isi.* FROM inventory_source_item isi LEFT JOIN catalog_product_entity cpe ON isi.sku = cpe.sku WHERE cpe.sku IS NULL;
Removing the ...
Based on magento documentation for flat table mentioned in https://docs.magento.com/m2/ce/user_guide/catalog/catalog-flat.html
Starting with Magento 2.3.0+, the use of a flat catalog is no longer a best practice and is not recommended. Continued use of this feature is known to cause performance degradation and other indexing issues
I believe this is because you use some custom module for FACT-Finder, and it disables Stores > Settings > Configuration > Catalog > Catalog > Catalog Search
Seams this isn't module; I installed it on my fresh Magento 2.3.3 and still have Catalog Search section visible. So for now, it's difficult to say what break the things.
Now I can propose you ...