Just because merchant has UPI scanner does not mean that UPI linked credit card will work. What I understand is this....
Merchants also have to enable accepting payment via credit card linked to UPI. It is an opt in. So for a merchant who has issues with MDR will not enable UPI Credit card option, he will only enable normal bank UPI.
I don't know how many of you have noticed phonepay scanner allowing credit cards. Some merchants allow card payment via phonepe scan (via phonepay saved cards) while some merchants only allow UPI.