I am in this situation where the owners group has been deleted from site collection owners and when i try to put it back I only get the members group available.
Can someone help? Is it a bug?
Yeah this can be done
If you navigate to https://(give your site URL)/_layouts/permsetup.aspx, you'll be taken to the Group Set-Up screen.
In the Owners section, use an existing group and see if you can find the Team Site Owners group in there. If so, select it and press OK. This should restore the group.
What's happened is that the site has removed the user group but it has't been deleted via the root site.
I also see O365 tag so do check this link here
Hope that helps,