Ok, just because you asked so nicely...
A week or two back I'd removed my wiim from amazon (unregistered it) and had uninstalled amazon music from my tablet, 'cos I'd cancelled amazon music months back, as mentioned.
Just now:
I reregistered with Amazon in the wiim app.
When I go to amazon.com.au, from my name in the top right i choose 'manage your content and devices' (not your apps and devices), click on the devices tab, click on 'other devices' and my pro shows up, registered date today.
Nothing about switching sites / contries.
I reinstalled the Amazon Music app, hit settings, top right, connect to a device, and all 3 pros were there as chromecast, and the one I'd just registered as alexacast.
Back into the wiim app, select a different pro, register with amazon, back into amazon music app, and now two pros are there with alexacast.
I'd say it seems pretty likely to be an amazon account setup issue.