Google Pay is now detecting and blocking devices with unlocked bootloaders.
I just went rounds with google support over my Mi9 not being accepted by Google Pay.
Their official response:
"I had escalated your case to our specialist team, I have received an updated. Our specialist found that your device bootloader is unlocked. I request you to please have the device bootloader locked. In order to use the Google pay app for the in-store purchase, the bootloader of the device needs to be blocked and also should have the NFC enabled on the device. Also make sure the Google pay is set as a default payment app. Please contact the device manufacturer to have the bootloader locked."