In order for URL tagging and post scheduling to work correctly NitroPack needs to be able to access your website directly, bypassing the caching layer provided by WP Engine. To make sure this is the case, please contact WP Engine's support team and ask them to configure a cache exclude based on the "nitropack-nocache" cookie. Feel free to use their chat support option as this is a really quick thing to do. Please phrase your request like this:
Hey, can you please configure a cache exclude based on the following cookie: nitropack-nocache
Once they confirm that the exclude is in place you should be all set for using NitroPack on WP Engine.