A bug in the Messages app means read receipts are being sent to senders, even when the feature is turned off in Settings.
Read receipts allow senders to determine whether or not a message has been read by the recipient. The status initially shows that a message has been delivered to another iMessage user, and the status changes to “read” when the recipient has viewed the message.
Some users prefer to have the feature toggled off, preferring to respond to messages on their own terms. Many senders may become offended if they see that a message has been read but not immediately responded to.
However, Macworld reports that there seems to be a bug affecting some users.
A number of people report that with Send Read Receipts disabled on their devices, people can still see when they read messages. This problem has cropped up seemingly briefly with previous releases of iOS and iPadOS, but I see a spate of reports with iOS 15, including the latest updates, in which people have a mismatch between their setting and other people being made aware of their status.
For now, it’s a bug with no particular resolution. Some people have found restarting their iPhone or iPad provides relief, at least temporarily.
If you want to check your own “Read” setting, look in Settings -> Messages, then scroll down to “Send Read Receipts”.
It’s unclear how many people are affected, but it’s likely that Apple will fix it in a coming update.