The default save button on a page will commit all entities on the page. If you have your own microflow performing the Save (commits) you need to commit all related entities (you wish to save) yourself.
So: retrieve by association (not via database) the associated objects, commit the lists (don't loop and commit in loop, this will be bad for performance).
Mendix will optimized the network calls. Since the objects are already part of the client state they will be sent to the server when executing the microflow. And the microflow actions (retrieve by assocation) won't need to access the database for those objects.
Well , if you find Microflows tricky to commit the objects
You can Nest all the things you want and just place “Save Changes” Button on the innermost Data view . This will automatically commit associated objects with the outmost Data view