Not sure why that is happening for you but it is unrelated to the Alexa ‘issue’ you are reporting, Perhaps try just refreshing on the Thing screen in the app.
Alexa is not really setting the fan to high and low per the device handler. It is interpreting ‘high’ as set higher and low as set lower, as if it were a dimmer. It does not have a medium equivalent it can recognize thus what you are seeing.
With Alexa you can tell it to set to a percentage. Different fans have different thresholds at which they run for low, med and high. In my case, all my fans run at low at a 33% setting, medium with 67% and high at 99%. So I can tell Alexa to set to 67% and it will set the fan to it’s medium speed.