Why do activity participants receive meeting invitations via email?

Issue:
When I create internal tasks, I sometimes need to add external contacts as activity participants. However, shortly after adding them, the external participants receive an automatic invitation email containing details about the activity (information that was intended to remain internal).

Current Setup:

• My Outlook is synchronized with Creatio.
• The Activities are of Category “To do.”
• Show in calendar is set to true.
• No reminders are configured.
• I do not click the “Send invitations” button.
• The “Send invite to participant” checkbox on the Activity Participant object is set to false by default.

Despite this, when external participants receive these unprompted invitations, I notice that the “Send invite to participant” checkbox on their Activity Participant record has automatically changed to true. This has occurred multiple times.

Question:
How can I safely add external contacts as activity participants to my “To do” activities without triggering invitation emails?