I’d try disabling Spotify in the Alexa app and relinking your Spotify account. Autoplay in the Spotify app itself shouldn’t impact Alexa use, but try turning autoplay off there too as well as in the Alexa app.
Does it do the same if you use the “message Alexa” voice input in the Alexa app, either to your phone, an echo or the WiiM?
Edit: it might also be useful if you screenshot the relevant part of the voice history in the Alexa app, showing both your request and Alexa’s response