Here is the reply I received from Nintex. The action is actually working as intended. The GUID variable is an Output, not an input. It outputs the item that changes on the list.
Hello Chadd,
I have had a chance to do further investigation, and although your's and the community member's findings are accurate, this is also what the action is designed to do. According to the documentation, this action "Delays the workflow until the selected event occurs in the selected list. " with the options being to wait for item addition or change.
https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fhelp.nintex.com%2Fen-US%2FO365%2FDefault.htm%23cshid%3D1128&data=02%7C01%7Ccsommerfield%40cardinalsolutions.com%7C168cac1a28bf4ac0922108d55a330164%7C414efc3368fe4520802faea4401192d0%7C0%7C0%7C636514097470591207&sdata=zkujZ%2BV8DZ7sKfy%2BJpsfC50NiFKxjh8iR7%2F7iRdaLvU%3D&reserved=0
This action pauses the workflow until an item is either added or changed on the selected list. The GUID of the list item that was either added or changed is output. This action is actually an out-of-box SharePoint Designer action, and operates as expected. Let me know if you have any additional questions.