[Feb 27th, See here for the latest update on the issue]
We have just identified a critical regression in February 2019 CU for SharePoint 2016:
Search queries from non-default zone URLs will not return results.
A fix for this is currently in the works but not yet available.
Customers are advised not to install February 2019 CU for SharePoint 2016.
A workaround os available for customers who have already installed February 2019 CU for SharePoint 2016 through Microsoft support.