E-mail Read/Unread indicators do not between Mac/iPhone/iPad

Are all of your devices configured to use IMAP or POP3 mailboxes? Most email Providers support IMAP - and many optionally support POP3. If any of your devices are configured to use POP3, the read/unread status may not synchronise between devices.

Unless you are sufficiently confident in manually checking and configuring your mail clients, the easiest method of resolving the problem is usually to delete the mail account from the device that is not properly synchronising with the others - and then re-add the account.

For your iPad or iPhone, to delete the associated mail account from your iPad:

Settings > Mail > Accounts > [Your mail account] > Delete Account

For Mac:

Log out of or remove email accounts in Mail on Mac - Apple Support

Having deleted the account, a forced-restart of the iPad/iPhone is recommended:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)

Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

Force Restart iPhone

Force restart iPhone - Apple Support

After restarting the device, you can re-add the mail account:

Add an email account to your iPhone, iPad, or iPod touch - Apple Support

Add email accounts in Mail on Mac - Apple Support

After re-adding the email account, the associated IMAP mailbox folders will reload from the server as they are accessed. This should have resolved your issue.