I am having the same problem with intermittent deliver of Alexa messages using the Alexa Switch in ST. The problem is definitely on the Amazon side.
We had similar problem 2-1/2 years ago using the old Groovy Virtual Alexa Switches became unreliable. So some of us switched over to using Voice Monkey to trigger Alexa routines. ST triggered an WebCore piston, the piston made and http call to Voice Monkey and Voice Monkey triggered and Alexa Routine.
The problem today is that WebCore is gone. I suppose SharpTools Premium or @TAustin Web Requestor (requires a constantly running server) could be used instead of WebCore to make the http calls.
Voice Monkey uses a virtual Door Bell to trigger the Alexa routines and it worked reliability. After several months Amazon fixed the problem and there was no reason to use Voice Monkey.
The Tasker “Auto Voice” plug-in also uses virtual Door Bells to trigger Alexa Routine.
@TAustin is any chance you could add a Virtual Door Bell with integral Switch to your virtual device creator. If this works it would be an easy solution.