iMessage went from showing what time it was read to just delivered?
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.