A steep and sudden drop in the email open rate usually suggests an issue with your DNS configuration and tracking, although this may not be evident automatically.
If you see this occurs, go to your Account Settings > click on Deliverability:
Then click on Verify your domain:
All records should display a green checkmark / Verified status.
If Invalid, this would in fact confirm an issue with the DNS setup resulting in the email open rate drop:
If you have any additional assistance from our Support team, please don't hesitate to contact us.