The problem doesn't seem to be the QR code itself, but rather the way KuCoin Pay is designed. With many crypto payment services, the QR code is only used to transmit payment information; final validation must then be performed within the official mobile app, which contains the required security keys, biometrics, or authentication mechanisms.
If the merchant requires KuCoin Pay, it's common for the following to happen:
You scan the QR code.
The browser opens a special KuCoin link.
This link attempts to launch the KuCoin app.
The app requests confirmation (transaction password, biometrics, passkey, etc.).
In this case, there is sometimes no mobile browser-only method to finalize the payment.
Some alternatives to check:
Open the payment page on a PC and use your phone only for validation.
See if the merchant offers a crypto address and amount to copy manually instead of KuCoin Pay. Check if KuCoin displays a button like "Pay with Web Wallet" or "Open in Browser."
Use a phone with more memory if the app has become too resource-intensive for your current device.
Regarding your comment about 3 or 4 GB of RAM, it's not necessarily the app size (178 MB) that's the issue. A modern crypto exchange app also uses a lot of memory while running, which can make the experience difficult on some entry-level phones, especially if multiple apps are open.
In theory, if KuCoin allows login via passkey in the mobile browser, it should be possible to confirm without the app. However, if the payment system requires redirection to the KuCoin app, the passkey alone is insufficient.