It has been over a month that I reported this issue and Nintex support acknowledge it as not just on my tenants, I am surprised nobody else has mentioned it here, and surprised that this issue doesn't' get moved to a higher priority.
Can others test on their environment please:
- create a simple Nintex Forms for Office 365
- create 1 item
- open it in display mode and click Delete in the ribbon.
Does it work? (it does nothing in our case and shows JS error in Developer Toolbar)