Many users prefer to turn off read receipts on the Messages app. That way, the sender won’t know if the message has been seen or not and the recipient, even though he’s seen it but hasn’t immediately responded, doesn’t have to worry about offending the other party. However, a bug on iOS and iPadOS caused the Messages app to send read receipts even if you turned it off.
Some users report that even if read receipts are turned off in Settings (Settings -> Messages -> Send Read Receipts), the device still sends this notification along with the time the message was viewed. This error has appeared on older iOS / iPadOS versions but not as serious as this time, according to MacWorld page, there are quite a few iOS 15 users facing the problem.
This is a small error, but it can affect the relationship of many people and unfortunately there is still no comprehensive fix. However, you can “fire” by restarting your iPhone or iPad. Unfortunately, this error may reappear later on affected devices.
At this point, rebooting seems like the only thing you can do to resolve the issue yourself, before Apple officially fixes the bug in future updates.
Reference: MacWorld
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