The «Magento\CatalogInventory\Observer\DisplayProductStatusInfoObserver» obverver declaration should be moved from «etc/events.xml» to «etc/frontend/events.xml» because it relies on the «Display products availability in stock on Storefront» config option

[dmitry_fedyuk (Dmitrii Fediuk)]

See How does Magento 2 decide whether to display the product’s Stock Status?

https://github.com/magento/magento2/issues/2352