Hello. I did a fresh install of UNA 12.1 and tried to reproduce the situation.
I created a paid cource. Pricing - Image 01
Course's card in browsing for viewer - Image 02
Cource view page for viewer - Image 03. It has both [Pay & Join] and [Attend Course] buttons. In this case the last one can be disabled via Navigation -> Courses -> View Actions. In future updates the last button is automatically hidden when the first one appears.