The till does not care that Gmail works. It cares that the QR resolves in a second. If WeChat and Alipay ride the VPN to Hong Kong, the scan stalls, the cashier asks you to try again, and the queue behind you decides you are the problem.
Chinese apps should stay on the local network while the VPN is on. That is the default. On Meridian, Smart Flow is the control if you need to change one app.
A tourist account on WeChat is already fussy about cards and verification. Do not add a slow exit on top of that. Pay on the local path. Browse on the tunnel.
This is the same split residents use, written up in keep WeChat and Alipay fast. If a payment fails, turn the VPN off for that one scan, pay, and turn it back on. Do not uninstall anything at the counter.