The Send Notification action sets the source URL of an image to be server relative rather than the full URL. This causes the server relative URL
For example, the HTML in the Send a Notification email body changes from:
<img src="https://[SharePointSiteURL]/Shared%20Documents/MyImage.jpg" />
to the following:
<img src="/Shared%20Documents/MyImage.jpg" />
Update to Nintex Workflow version 126.96.36.199 (April 2021 release)
This issue was identified as a defect in Nintex Workflow for SharePoint 2019 and has been addressed in version 188.8.131.52.