I have process model, after start node configured a user input task with MNI and activity chained. I have configured this process model as related action when I initiates the process from the record it is not showing the form instead it shows action completed
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MNI and chaining are basically mutually exclusive. Chaining says "keep the user with this one sequence of events"; MNI says "run multiple copies of this". So MNI nodes usually break chains, since you can't keep a user with a single process flow when you create more than one of them.
In this example, what I'd expect to happen is the user gets the "completed" message, but has multiple tasks assigned to them in their task queue, each of which allows them to fill out the user input task.