I've had this happen several times with the Stadia app on iOS - I go to buy a game, and my stored card says that it's been declined. I know that that card shouldn't be declined, so I try adding it again. When I add the exact same credit card again (with the same information), the transaction goes through. This has happened twice, and when I mentioned this happening on Reddit, at least one other person replied and said the exact same thing had happened to them. (I know that that's anecdotal, but at least I believe it's not just me.)
Hi, This doesn't happen to me, but, I suspect that the issue's on the banks transactional system. It could be coincidental. Basically, the first request times out, and Google interprets the answer as a decline. The second request doesn't time out, and returns as accepted. Contact your bank you keep experiencing this issue.