I suggest that you use a tool like Fiddler to check the traffic that the dashboard is sending. I suspect that it is not sending the prompt value you think it is.
I can’t be sure without Fiddler logs, but that sounds a lot like the web service is firing off once with bad prompt values, and then again with the correct prompt values. I’d check the cells where you are getting your prompt values, and the cells you are using for your triggers.