Managed to use it yesterday from my journey to the east.
I've noticed something when I'm using Google Pay. It takes the machine time to toggle between different modes before going to detect bank cards. How do I know this, on Android devices it will vibrate when it is being scanned or sent the signal to the reader. It vibrates twice before the gate detecting the bank card all this within a second or a second half.
As far as I know, there are 3 kinds of cards being supported, one being the bank card the other two is CEPAS and the phasing out EZ-Link. So the latency of scanning is highly likely due to this switching.
Secondly, when you register your card with ABT on Mobile, it takes time for it to propagate your account across different terminals and services. MRT gates are via LAN and should be the fastest and the busses only do so around the Bus terminals where they have a WiFi network to sync various of things between the busses local database to the servers. (I can't elaborate on this..Potentially P&C)
I guess that could probably be the reason my watch sometimes show payment completed but no respond on the reader....