Sounds like you set the subform to Data Entry = Yes rather than the main form
Setting data entry to Yes opens the relevant form to a new blank record.
For example, main form with data entry = yes, subform with data entry = No
View attachment 108832
This time, main form with data entry = Nos, subform with data entry = Yes
View attachment 108833
I'm not sure why you'd ever want to do either of these though