In Version 5.20. DropDown control show previous value if bound value is null or undefined

[baminmru]

I'm submitting a...

[X] Regression (a behavior that used to work and stopped working in a new release)
[X] Bug report  
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request

Current behavior

Case 1:
I use same form for edit and create new records.
I open record for edit. I close form, and after open form for new record. All Dropdown controls show values from edited record. Controls not empty. And I see some values in dropdown. If now I save new record values in record ( in data base) stay empty.
Case 2:
If I edit record where bound field is set, I see right values at dropdown.
If after that I open form for record where bound values is null, I see values from previous record.

Expected behavior

Dropdown clear text iv bound value is null or undefined

Minimal reproduction of the problem with instructions

For bug reports please provide the STEPS TO REPRODUCE and if possible a MINIMAL DEMO of the problem via
https://plnkr.co or similar (you can use Our Amexio Plunker as a starting point: http://plnkr.co/edit/M9AeGH?p=info)

What is the motivation / use case for changing the behavior?

Environment

Angular version: 6.1.9


Browser:
- [ x] Chrome (desktop) version 80
- [ ] Chrome (Android) version XX
- [ ] Chrome (iOS) version XX
- [ x] Firefox version 75
- [ ] Safari (desktop) version XX
- [ ] Safari (iOS) version XX
- [ ] IE version XX
- [ ] Edge version XX
 
For Tooling issues:
- Node version 12.16.0 
- Platform:  Windows 

Others:

[baminmru]

drop down work ok in 4.1.3

[baminmru]

This problem is returned back.

[baminmru]

Here is screenshots of edit form with right dropdown values, and wrong values if bound fields is null.