I sent an iMessage and it showed it was read at 18.01. Few hours later it just says delivered and the message i sent before the last one shows the time it was read. Why did it go from read at 18.01 to just delivered? Have i been blocked by this person or what?
I have seen this too on my own device. It is a bug, though it has persisted through several iOS 9 releases. (I think it started with 9.3 but I'm not positive on that.) It does not affect read receipt display on Messages for Mac.
Read receipts are not retroactive, so even if you were blocked or the other person changed their read receipt setting, this would not occur. If you watch, when a future message is received, you will likely see that the last read receipt shown will move forward one message, to the message immediately before the latest message (in other words, the one where it is not displaying right now).
You can use the iPhone feedback tool to report this bug to Apple.
From an article on the subject:
however it’s also important to note that this setting only applies to the current device. If you’re using iMessage across multiple devices — such as on a Mac and/or iPad — you’ll need to turn the setting OFF individually on each of your devices if you don’t want read receipts sent from those devices. Note that users will still see that messages have been “Delivered” — there’s no way to turn this one off