Nice to have you in the Community. I'm here to lend a hand with terms calculating not calculating correctly on your invoices.
QuickBooks Online offers the ability to customize terms, such as specifying a set number of days or time of the month that the invoice is due. Since you've referenced that it will autofill to the end of the month, it sounds like the terms may have been customized. Here's how you can view and edit the terms:
Select the Gear icon.
Click All Lists.
Select the triangle drop-down under the Action column and click Edit.
Verify that the Term is properly configured.
The following is a brief video to serve as a visual reference: