As I understand it, this isn't a bug.
BETA software isn't recognises as official authenticated software, therefore some of the security certificates aren't signed, hence no Samsung pay.
Before I decided to opt in it was clear that we can opt out easy using downgrade option. Unfortunately it is not visible at Smart Switch.
Here is an official information from Samsung members app:
10. How can I restore the official software during the Beta Programme?
You can re-install the official software by using the PC version of Smart Switch.
(You can download it from the following link - http://www.samsung.com/uk/getting-started/smart-switch/
Please connect your phone to your computer, launch Smart Switch on your computer, and press the “Downgrade” button to re-install the official software (rollback).
Don't you just hate a smart *****? I'm sure we're all aware that enrolling in a beta program is all about finding bugs. You make a decision about enrolling based on previous and reported experience. By the time a beta gets to the stage where it is rolled out to anyone who wants to participate the bugs are mostly ironed out. Previous Samsung public betas have been pretty solid. This one is very different. This beta was just too buggy for me and I've particpated in a great many beta programs in my time.