There isn’t anything built into ACF that will do this and what you are seeing is the expected behavior.
You have 2 choices.
1) You build an “if” statement in your template that checks the value of the field A and then displays the values if it is checked.
2) Build an acf/save_post filter https://www.advancedcustomfields.com/resources/acf-save_post/ that checks the value of field A and when it is not checked deletes the values from the database.
