Hello,
The only thing that’s changed with respect to when the UI is shown, is the new capability. The plugin checks to see that the current user has the tip_the_author capability and that the author has the receive_tip capability. So first ensure that the capabilities are assigned to the users. If you assign the capabilities to the user roles, check to see if the capability is assigned to all the user roles for your users, as a user can belong to multiple roles.
Check the above and let me know if you are still encountering this issue.
with regards