I have a Nintex workflow in our On Prem SharePoint 2016 farm that with some actions configured inside a "Run If" action. The run if is configured to run if a multivalue choice field is not empty. The field does contain data (verified by outputting to a log to history action), but yet the actions still do not run.
Can you provide some screenshots?
I tested it and it works fine.
"Auswahl" is a multiple choice field.