Topic
The Nintex Forms and Workflow icon disappears from the ribbon.
Instructions
This behavior is by design from SharePoint specifically for 2019. Nintex Forms and Nintex Workflow buttons are added as custom action buttons on SharePoint ribbon. Nintex has added the Registration ID and Registration Type in the Custom button XML to make it work for the Modern and Classic view in SharePoint 2019. Since these details have been added, the custom action button for the classic view under no toolbar is not visible.
As per the default SharePoint 2019 behavior, Custom Action buttons added using Registration ID and Registration Type in XML will not be visible on the ribbon upon selecting the option as No toolbar on the web part.