Multiple emails in a single field not working for sendgrid "To" field

Column C combines 2 fields (Column A&B) that contain email addresses. When I select Column C as the main email recipients in Sendgrid app, it sends separate emails to each individual in column C. However, if I select column C to cc: my emails, it correctly sends a single email to all recipients.
I would like ONE email to be sent to all the people in column C in the “To” field in sendgrip.
Can anyone explain why this is happening and help me find a solution?

Thanks!