Nb radio button dynamic form problem

### Issue type

**I'm submitting a ...** (check one with "x")

* ["x" ] bug report
* [ ] feature request

### Issue description

**Current behavior:**

In dynamic form when I checked the radio button first form it checked and after I move on to second form the radio button checked and the first form radio button has unchecked but both radio button value has been got. only the problem check and unchecked.

**Expected behavior:**

I am excepting both form radio button should checked
this demo will show the expected behavior
https://stackblitz.com/edit/mat-radio-button-example-rl6yct?file=src%2Fapp%2Fapp.component.html
**Steps to reproduce:**

**Related code:**
please check this example:

https://stackblitz.com/edit/github-xitqcx-jfhqzb?file=src%2Fapp%2Fapp.component.ts

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