Put in a ticket through their website and got the following response. Updating worked for me, but I did have to go back in to the container tags and re-enter the tag.
I am sorry for the trouble. Custom Class names that contained spaces were inadvertently broken when patching a possible security issue with the Custom Class name fields. Can you update Ninja Forms to version 3.5.8.3 and let us know if you are still seeing the same issues?