Change content type for an item is not supported
If document libraries are associated to content types and are being customized using Nintex forms, then change in content type value is not working. Once your document is moved to a content type where it has nintex form associated, it won't allow you to change content type. To reproduce the issue
1. Create 2 content types and customize forms using Nintex
2. Assign these content types to 2 document libraries
3. Add docx files to the library and set documents to one of the content type
4. Change content type of the document that you uploaded
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Yes, they are working to fix it to sharepoint 2013/2016.
Did Nintex support say if the issue in 2013/2016 is being addressed?
Nintex support tell me, the problem is docx file to sharepoint 2013/2016, to Sharepoint 2019 the issue is fixed. You can make a test with doc file and it works. I have the same issue.