I must thank those that insisted on the anomality around the 'matrix' gem.

The resolution is as follows:

1. for an unknown reason, 'bundle update' didn't identified that 'matrix' is required by other gems, such as 'roo' which are active in production. Heroku removed it because it was marked for dev/test.

2. I've introduced 'matrix' directly on my Gemfile and 'bundle update' marked it for prod as well.

3. Push was successful.