I have uncovered a strange problem to do with the new item button on SharePoint lists (now in the Microsoft Lists view format). This Microsoft feature release has also caused other issues with list views using JSON customisations, but until recently, nothing has caused issues with the core functionality.

So, what has happened is that I have created a new blank list, enabled management of content types and assigned a site content type to it. Set-up the view and tested. All good. Now, I usually then go back to the settings of the list and hide the default ‘Item’ list content type or just delete it from the list.

My added site content type will be the basis for a form, so I launch the Nintex Forms app and choose my content type, make some small edits to the layout and save/publish it. Then I usually go to the new item button and expect to see the Nintex form open for input in New mode. However, now this is not happening. Instead, I get the default SharePoint list form for the default ‘Item’.
Now, I know SharePoint online caching can be insanely aggressive and holds on to cached values through several deep refreshes, but this issue remains.
If I unhide the default Item, then refresh, I get a choice of my content type and the default item. If I select my content type, then Nintex duly opens for input. As soon as I either hide the default list content type again, then I cannot open my content type using the new item button, but again, the default. If I go one step further and delete the list content type, leaving just mine. I get my form, but rendered in the default SharePoint list form and not Nintex.

The only way I can get it to work as expected is to force the entire list into Classic view, where the new item button launches the Nintex form normally. But that is not really an acceptable solution. Has anyone else experienced this? And of course, is there a fix?