Garmin Pay - I need to re-enter PIN sevaral times in the day

Check for gaps in heart rate data. If the watch cannot detect your heart rate intermittently, it might think it's off wrist.

Don't seems....but if there is a micro disconnection is verry difficult to verify. Thank you

The use of Garmin Pay depends on wrist detection. The idea is that the PIN stays valid as long as the watch detects it is still on your wrist. This is a safety feature imposed by regulators and banks to make sure its you paying for the transaction. If it loses the detection of your wrist (too loose wearing it or outside influences) it will ask to enter the PIN. It is not exactly detecting heart rate, it is detecting skin reflection and responds within 1-2 seconds with a lock, you might not see that in the heart rate registration even. Of course skin tone has impact in the reflection and is know to be a factor in the detection effectiveness. Oh, and it is built in to enter the PIN every 24 hours based on the same safety feature.