Magento2: Magento 2 : Thumbnail Role Missing In Product EDIT page admin

[nishantbkw]

Update : Now Upgraded to 2.1.8 before it was 2.1.1

Preconditions

1. Magento 2 version 2.1.8

ISSUE

I am trying to figure out why thumbnail role suddenly missing when i try to edit a product from admin and want to change thumbnail image also for new products thumbnail role not showing please see attached image

Steps to reproduce

1. From Magento 2 admin go to products->catalog try to edit any product
2. Click Images and Videos Tab
3. Select any image which i want to assign as thumbnail image a popup appears with roles but thumbnail role is not here also same happens for add new product see attached image for details

Expected result

1. In the Popup Roles section thumbnail role should appear as it used to show here but suddenly stopped showing for unknown reason

Actual result

1. Thumbnail Role Is Missing It used to be here before
   

[orlangur]

2.1.1 is quite old, can you reproduce this issue on latest stable 2.1.8 version?

[nishantbkw]

Ok , now i upgraded to latest version 2.1.8 but it does not fixed the issue please assist me
thank you

[nishantbkw]

Solved!
the Attribute "thumbnail" was not assigned to "Default" attribute set now i manually assigned it through backend and it started appearing in Role section.
Not sure how it suddenly unassigned from Default attribute set