If I get the time, perhaps I will dig into and confirm whether there is a bug here preventing the default from propagating.
In the meantime, via some code-reading, I learned that if you add the following to the plugins.js file (in the config: {} section), then this starts to work again! What an excellent setup! strapi has really got some impressive features!! 
breakpoints: {
large: 1000,
medium: 750,
small: 500,
},
So my complete config setup:
upload: {
config: {
provider: 'strapi-provider-upload-aws-s3-plus-cdn',
providerName: 'strapi-provider-upload-aws-s3-plus-cdn',
providerOptions: {
providerName: 'strapi-provider-upload-aws-s3-plus-cdn',
accessKeyId: env('AWS_ACCESS_KEY_ID'),
secretAccessKey: env('AWS_ACCESS_SECRET'),
region: env('AWS_REGION'),
endpoint: env('AWS_ENDPOINT'),
params: {
Bucket: env('AWS_BUCKET'),
},
cdnUrl: env("CDN_URL"), // Optional CDN URL - include protofol and trailing forward slash, e.g. 'https://assets.example.com/'
},
breakpoints: {
large: 1000,
medium: 750,
small: 500,
},
},
actionOptions: {
upload: {},
uploadStream: {},
delete: {},
},
},