I noticed that you created a Support ticket for the same issue. Please continue working with our Support Team. Once it is closed/solved, I will add the solution to this topic so it can help other folks in our community in the future.
Here is the updates for the cause of the issue and the solution.
The “Display Connection as a Button” option is visible for the New Universal Login experience, not for the Classic Login Experience. Also, please make sure Login, password Reset, and Multi-Factor Authentication customization are disabled.
@lihua.zhang - I double checked my tenants’ settings and it looks the same in both the tenants but still I see only one tenant showing this option and other doesnt. Could you please provide more documentation on exactly what steps to be followed if its missing.