In this case, as it might be a monitor problem with your second monitor, I would recommend to ask your manufacturer if there's a problem or you could also leave feedback in the Stadia App, to make the developers aware of an issue with Stadia on that specific monitor. Normally you don't get an answer to your feedback, but it's reviewed by the developers and then they can take action on the problem or address this.
But if you only have this problem with that specific monitor, might be also that this is not working properly with Stadia in general (who knows why).
Hope the best that it'll be fixed soon for you!