When I was playing with observers in the snapshot, I discovered that locking/unlocking the repeater will be detected by the observer. I first discovered that in 17w50a, and I tested in 17w47a. In 17w47a it would also update the observer. I think that it may be caused by the flattening.

This bug is not mentioned on neither minecraft.net nor Minecraft wiki as a "feature", so I reported it. However, this bug is rather interesting and quite useful, and I love it so much. I have created some useful stuff with it. For example, you can create a switch that give two pulses when turn on and only one pulse when turning off in just a 2*3 area with two repeaters and one observer.

Please make it a feature. But if you don't want to, fix it anyway.