The minor versions 3.0.3 and 3.0.4 are missing from the CDN. They return an HTTP 404.
I don't know what they do via the CHANGELOG but it seems that they might be using a new buy sdk version which might be important.
https://sdks.shopifycdn.com/buy-button/3.0.3/buybutton.js NOT WORKING, 404
https://sdks.shopifycdn.com/buy-button/3.0.4/buybutton.js NOT WORKING, 404
https://sdks.shopifycdn.com/buy-button/3.0.2/buybutton.js WORKS!