Apple Mail app not updating from unread to read emails on iPhone

Depending on how long ago and exactly how your Yahoo account was added to the devices can make a big difference on how smoothly those accounts work. For example, in the account on the iPhone could be using an old protocol like POP3 instead of IMAP. The methods used for authentication can also change over time and the server addresses that a provider wants you to use can also change over time. And lastly, it is always possible that the database that actually stores the email content has some corruption.

For all of these various reasons, deleting an account from a device, rebooting the device, and then adding the account back is the tried and true method (regardless of the device or OS) that ensures that an account has a freshly created mailbox database, is using the currently recommended server settings, and is using the most modern authentication (and 2FA). You may want to also consider doing the same procedure on any device that is misbehaving so that all of them are similarly using modern settings.