This is what I'm seeing:
I create a new Trial environment, with database, then assign users with Environment Maker roles. Those users cannot access the environment (in make.powerapps.com the environment is not available as a selection). I have attempted assigning all roles to these users with no difference.
If I create a new Trial environment, skip the database, then assign users with Environment Maker roles, then create database, those users are able to access the environment. I have noted the roles those users were automatically assigned and verified they were included in the roles I assign manually when creating the database when creating the environment.